docs: rename core-data-structures/ to subsystems/

The folder is becoming the home of one-doc-per-subsystem pages (intro +
data structures + cordis services/events), so the name must describe the
whole contract, not just the type-vocabulary third of it. Mechanical
rename rebuilt on current master: every inbound Markdown link, generator
constant, website route, type-equiv manifest path, and spec expectation
moves together; the zh sides of the notes whose prose names the folder
are aligned (子系统) in the same change; touched bilingual pairs
re-recorded; translation-prompt snapshot re-recorded (its example embeds
development.md). Historical Agent Note slugs keep their dated
filenames.

docs/subsystems/sandbox.md

@@ -0,0 +1,148 @@
+# Process Sandbox
+
+English | [中文](sandbox.zh.md)
+
+The process-sandbox seam of [dsh-sandbox](../../packages/sandbox/sandbox) wraps a same-world subprocess argv in a file-effect policy without coupling consumers to a platform runner. [dsh-sandbox-local](../../packages/sandbox/sandbox-local) supplies the Linux bwrap/Landlock and macOS Seatbelt backends; [dsh-bash-sandbox](../../packages/bash/bash-sandbox) is the first consumer. Containers, microVMs, and remote execution are sibling implementations of whole capability seams, not providers of `ctx.sandbox`.
+
+Source: [`packages/sandbox/sandbox/src/index.ts`](../../packages/sandbox/sandbox/src/index.ts)
+
+## Modes and enforcement
+
+`SandboxMode` governs filesystem effects only. `read-only` denies writes except the required `/dev/null` sink; `workspace-write` permits writes under the workspace root and the backend's promised temp area; `danger-full-access` bypasses confinement. Network and process visibility are outside this vocabulary.
+
+```ts type-equiv
+/**
+ * File-effect policy for confined processes. `read-only` permits only required
+ * sinks such as `/dev/null`; `workspace-write` also permits the workspace and a
+ * backend-defined temp area; `danger-full-access` bypasses confinement. Network
+ * and process visibility are outside this vocabulary.
+ */
+type SandboxMode = 'read-only' | 'workspace-write' | 'danger-full-access'
+```
+
+Only the first two modes can be sent to a provider. A `danger-full-access` consumer spawns its original argv and does not call `ctx.sandbox`.
+
+```ts type-equiv
+/** A confining (non-`danger-full-access`) mode — the modes a {@link SandboxPolicy} can carry. */
+type ConfinedSandboxMode = Exclude<SandboxMode, 'danger-full-access'>
+```
+
+Enforcement is a reported fact. `full` means the backend governs every file effect promised by the mode; `partial` means an active backend or older kernel ABI governs only a subset, so consumers that require the absolute promise must reject or surface that distinction.
+
+```ts type-equiv
+/**
+ * Enforcement completeness for this host. `partial` means an active backend or
+ * older kernel ABI cannot govern every promised file effect; callers requiring
+ * an absolute boundary must not treat it as `full`.
+ */
+type SandboxEnforcement = 'full' | 'partial'
+```
+
+## Per-call policy
+
+The complete execution policy is resolved and carried per capability call. It includes `danger-full-access` so a consumer can resolve policy once before deciding whether to bypass confinement. Normal tool calls derive `workspaceRoot` from the calling session's immutable cwd; deployment configuration is the agentless fallback. The root is canonicalized with filesystem semantics before lexical normalization, so a cwd containing `symlink/..` identifies the directory where a spawned process actually runs.
+
+```ts type-equiv
+/**
+ * The complete file-effect policy resolved for one capability call. The root
+ * is carried even under modes that do not consume it so callers can resolve
+ * policy once before choosing the enforcement path.
+ */
+interface SandboxExecutionPolicy {
+  /** The file-effect mode this execution runs under. */
+  mode: SandboxMode
+  /** Absolute root directory `workspace-write` may write under. */
+  workspaceRoot: string
+}
+```
+
+`ctx.sandboxPolicy.resolve()` accepts the active session and, for an approved retry, an explicit mode. The service owns precedence and root fallback so bash and fs do not repeat it.
+
+```ts type-equiv
+/** Inputs that select the sandbox policy for one capability call. */
+interface SandboxPolicyRequest {
+  /** Calling session; its immutable cwd becomes the workspace boundary. */
+  session?: Session
+  /** Explicit approved mode override, which outranks session policy. */
+  mode?: SandboxMode
+}
+```
+
+Only a confined execution reaches `ctx.sandbox`; its provider policy narrows the mode while retaining the same root. This permits concurrent sessions, consumers, and one-shot escalated retries to ask the same provider for different boundaries without mutating provider state.
+
+```ts type-equiv
+/**
+ * What one confined execution is allowed to touch — carried PER CALL, not
+ * fixed on the provider: two consumers may confine under different policies
+ * at the same instant (bash under `read-only` while a confined child agent
+ * needs its state directory writable), and an approved escalated retry is a
+ * new call with a wider policy. Defaulting/resolution is an explicit step at
+ * the consumer boundary; the provider treats the policy as fully specified.
+ */
+interface SandboxPolicy extends SandboxExecutionPolicy {
+  /** The file-effect mode this execution runs under. */
+  mode: ConfinedSandboxMode
+}
+```
+
+## Wrapped argv and classification dialects
+
+`RunnerFailureRule` combines evidence that a runner failed before executing the command. A consumer requires a nonzero exit, the optional allowed-exit-code gate, and a case-insensitive fatal signature within one remaining stderr line. Case-insensitive exact full-line informational exclusions are removed first, so a benign runner notice cannot prove failure by itself. The matched line remains available as error detail; classification does not rewrite stderr.
+
+```ts type-equiv
+/**
+ * Evidence that identifies a sandbox runner failing before it executes the
+ * wrapped command. A consumer first applies {@link allowedExitCodes} when
+ * present, removes {@link informationalLines} by case-insensitive exact line
+ * equality, then matches {@link fatalSignatures} case-insensitively within
+ * each remaining stderr line. Exit status alone never proves runner failure.
+ */
+interface RunnerFailureRule {
+  /** Nonzero process exit codes on which this rule may match; omitted permits any nonzero exit. */
+  allowedExitCodes?: readonly number[]
+  /** Non-empty substrings identifying a fatal runner diagnostic on one stderr line. */
+  fatalSignatures: readonly string[]
+  /** Benign stderr lines excluded by exact full-line equality before fatal matching. */
+  informationalLines?: readonly string[]
+}
+```
+
+`ConfinedArgv` is what the consumer spawns. Besides the replacement argv, it carries the backend's enforcement fact and two orthogonal stderr classifiers. `denialSignatures` identify the confined command being blocked while the sandbox works correctly. `runnerFailureRules` identify the sandbox runner refusing or failing before it executes the command; consumers check these first and surface a sandbox infrastructure failure, never an ordinary task failure.
+
+```ts type-equiv
+/**
+ * A {@link SandboxProvider.confine} result: the argv to spawn in place of
+ * the caller's own, plus the enforcement completeness the selected backend
+ * achieves for it.
+ */
+interface ConfinedArgv {
+  /** The wrapped argv (runner, profile, separator, then the caller's argv). */
+  argv: string[]
+  /** How completely the selected backend enforces the policy's file effects. */
+  enforcement: SandboxEnforcement
+  /**
+   * The selected backend's denial DIALECT: the case-insensitive stderr
+   * substrings a file effect denied by THIS backend produces (EROFS text
+   * under bwrap's read-only binds, EACCES under Landlock, EPERM under
+   * Seatbelt). A consumer that infers denials from a failed run's stderr
+   * matches against exactly these rather than a cross-backend union — the
+   * union claims denials a given backend never produces.
+   */
+  denialSignatures: readonly string[]
+  /**
+   * Structured runner-failure evidence rules. Consumers require a matching
+   * fatal stderr line (after informational exclusions) and any rule-specific
+   * exit-code gate before checking denial signatures: runner failure means the
+   * command never ran, while denial means confinement worked and blocked it.
+   */
+  runnerFailureRules: readonly RunnerFailureRule[]
+}
+```
+
+The [local provider](../../packages/sandbox/sandbox-local/README.md) owns operator configuration and maps its runner dialect into these rules. The [sandboxed bash consumer](../../packages/bash/bash-sandbox/README.md) owns spawn and result attribution.
+
+## Provider and fail-closed errors
+
+`ctx.sandbox.confine(argv, policy)` returns a `ConfinedArgv` or throws `SandboxUnavailableError` with code `SANDBOX_UNAVAILABLE` when no usable backend exists. Consumers may also classify a failure while spawning or observing the returned argv; that attribution belongs to the consumer contract. Silent unconfined passthrough is never legal for a confined policy.
+
+Provider selection, probing, caching, and backend-specific enforcement reports belong to the [local provider](../../packages/sandbox/sandbox-local/README.md).