feat(sandbox): cross-family file sandbox — one policy home, sandboxed fs provider, fs escalation parity

Extend SandboxMode enforcement from bash to the filesystem tools, the sandbox
RFC's deferred cross-family phase.

- dsh-sandbox-policy (new, ctx.sandboxPolicy): the single home for the
  deployment default mode + workspaceRoot and the per-session override event,
  renamed bash/sandbox-mode -> sandbox/mode and moved here with its fold/setter.
  Decouples the bash seam from dsh-session.
- dsh-fs-sandbox (new): SandboxedFileSystem extends LocalFileSystem and fences
  write/edit by the per-call mode (read-only denies, workspace-write contains to
  the workspace + temp roots via the shared writableRoots, danger passes
  through); reads pass through. Structured FS_SANDBOX_DENIED; in-lock parent
  re-canonicalization. A policy fence in trusted code, not a kernel boundary.
- dsh-sandbox: the shared escalation kit (writableRoots, the strictly-wider
  ladder, denial/hint markers, approveEscalation) both tool families use;
  approveEscalation takes a structural approver so dsh-sandbox gains no
  approval/agent dependency, and both tools stay duplication-free.
- tool-fs: write/edit advertise sandbox_permissions/justification under a
  confining ctx.fs, map FS_SANDBOX_DENIED to the shared [sandbox: ...] marker,
  and resolve the same one-approved-wider retry.
- examples/acp-agent: composes sandbox-policy + fs-sandbox, drops the gating
  that disabled the fs stack under confined modes.

RFC docs/rfc/implemented/feature/2026-07-14-cross-family-fs-sandbox.md; the old
sandbox RFC's In-process/deferred/FAQ sections updated to shipped fact.

docs/architecture.md

@@ -26,6 +26,7 @@ A harness is one [Cordis](cordis-primer.md) context. Packages contribute service
 | `ctx.llm` | [`llm/`](../packages/llm/README.md) | adapter registry and streaming model calls |
 | `ctx.bash` | [`bash/`](../packages/bash/README.md) | foreground/background command execution |
 | `ctx.sandbox` | [`sandbox/`](../packages/sandbox/README.md) | same-world process confinement (argv wrapping, per-call policy) |
+| `ctx.sandboxPolicy` | [`sandbox/`](../packages/sandbox/README.md) | shared sandbox policy: mode, workspace root, per-session override |
 | `ctx.codeRuntime` | [`code-runtime/`](../packages/code-runtime/README.md) | model-written program execution |
 | `ctx.fs` | [`fs/`](../packages/fs/README.md) | filesystem provider primitives and policy events |
 | `ctx.skills` | [`skill/`](../packages/skill/README.md) | skill provider registry and progressive disclosure |