feat(modes): per-session sandbox/approval switching — the session log as the store, ACP config options

effective(session) = findLast(the session own knob events)?.value ?? the
composition-config default. One log-only event per knob, owned by its
domain (bash/sandbox-mode in dsh-bash, approval/policy in dsh-approval),
each exporting the same three-piece kit: the event declaration, a pure
fold, and THE write path — a switch IS its event; no owner service, no
facts map. Restart immunity and multi-session isolation fall out of the
log replay by construction.

Execution follows the fold on both sides: the bash tool stamps
escalation grant > session override > executor default, and the approval
seam prepends the never-gate that auto-rejects before any interactive
answerer. Visibility is two layers per knob: a per-agent prompt section
states the effective value on every request (logged through
request/header*, so what-the-model-was-told replays from the log), and an
agent/pre-step narrator injects at most one coalesced delta notice with
positional attribution (user switch vs operator/config drift). The ACP
bridge advertises one capability-gated select per composable knob with
currentValue folded per session, validates set_config_option against the
closed vocabularies, and anchors idle switches at the next turn
prompt-submit under the turn-enclosure contract.

packages/bash/tool-bash/README.md

@@ -17,7 +17,7 @@ The plugin also contributes the `tool:bash` prompt section (order 105) — the c
 | `timeoutMs` | number | Timeout override in milliseconds. The executor applies its configured default and cap. |
 | `workdir` | string | Working directory for this call. Defaults to the calling agent's session cwd (`session.header.cwd`) so each session runs in its own workspace; a relative `workdir` is resolved against that session cwd. |
 | `run_in_background` | boolean | Return a task id immediately; no timeout applies. |
-| `sandbox_permissions` | string enum | ADVERTISED ONLY when the mounted executor sandboxes (`ctx.bash.sandboxMode` reports a confining default): the strictly wider mode a denied command needs (`read-only` offers `workspace-write`/`danger-full-access`; `workspace-write` offers `danger-full-access`; nothing above `danger-full-access`, so the fields vanish). |
+| `sandbox_permissions` | string enum | ADVERTISED ONLY when the mounted executor sandboxes (`ctx.bash.sandboxMode` reports a confining default): the wider mode a denied command needs, from the closed target vocabulary `workspace-write`/`danger-full-access` (never cut down to the executor's default — the effective mode is per-session; strict widening is checked at execution against it, and a non-widening request fails without prompting anyone). |
 | `justification` | string | Required together with `sandbox_permissions` (each without the other is a validation error): one sentence for the user explaining why this exact command needs the wider access. |
 
 `command`, `workdir`, and `timeoutMs` are resolved against the executor's config defaults via `ctx.bash.resolve()` before execution, so the executor seam (`BashExecSpec`) receives explicit `workdir`/`timeoutMs` values. The workdir default is applied in the tool layer (from the calling agent's `session.header.cwd`) BEFORE `resolve()` — the per-session cwd must come from `exec.agent`, since N sessions share one executor; only when no session cwd is available does the executor fall back to its own config / `process.cwd()`.
@@ -52,5 +52,8 @@ The `BashExecRequest` seam carries optional `stdin` and `env`, used by the hooks
 
 Commands run with the executor's full authority unless a sandboxing executor ([`dsh-bash-sandbox`](../bash-sandbox/)) confines them — the deny-only sandbox reports denials as result facts, rendered here as the denial marker; per-call allow/deny/ask policy is the `tools/pre-execute` waterfall (see docs/architecture.md).
 
-On top of a denial sits the escalation gate ([the sandbox RFC § Escalation](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md)): an escalating call (`sandbox_permissions` + `justification`) resolves [`ctx.approval`](../../approval/approval/README.md) BEFORE anything executes — `allowed-once` stamps the granted mode onto the bash request as the seam-level `sandboxMode` override (that one call runs, classifies, and reports under the wider mode; its neighbors keep the executor's default), while `rejected`/`cancelled`/`unavailable` and the no-service / no-agent paths each fail closed with their own error text and execute nothing. The seam is consumed opportunistically (`ctx.get('approval')`, the dsh-tools ask-routing pattern); the grant is consumed by the very call that asked, and nothing is stored. The static description teaches — and a denied result itself prompts, via the escalation-available marker appended exactly when the fields are advertised — the SAME-TURN flow: on a denial a wider mode would cure, retry the exact command once with `sandbox_permissions` (the narrowest mode that suffices) + `justification` immediately, without detouring through chat (the approval prompt IS the user's consent); never speculatively — an escalation is grounded in a real denial (up-front only when the session already denied the same access), a prompt-stated approvals-disabled policy turns the exception off entirely, and a rejected escalation is final for that command.
+On top of a denial sits the escalation gate ([the sandbox RFC § Escalation](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md)): an escalating call (`sandbox_permissions` + `justification`) resolves [`ctx.approval`](../../approval/approval/README.md) BEFORE anything executes — `allowed-once` stamps the granted mode onto the bash request as the seam-level `sandboxMode` override (that one call runs, classifies, and reports under the wider mode; its neighbors keep the session's effective mode), while `rejected`/`cancelled`/`unavailable` and the no-service / no-agent paths each fail closed with their own error text and execute nothing. The seam is consumed opportunistically (`ctx.get('approval')`, the dsh-tools ask-routing pattern); the grant is consumed by the very call that asked, and nothing is stored. The static description teaches — and a denied result itself prompts, via the escalation-available marker appended exactly when the fields are advertised — the SAME-TURN flow: on a denial a wider mode would cure, retry the exact command once with `sandbox_permissions` (the narrowest mode that suffices) + `justification` immediately, without detouring through chat (the approval prompt IS the user's consent); never speculatively — an escalation is grounded in a real denial (up-front only when the session already denied the same access), a prompt-stated approvals-disabled policy turns the exception off entirely, and a rejected escalation is final for that command.
 
+## Per-session mode switching
+
+Under a sandboxing executor this plugin makes the session's standing mode override ([the sandbox RFC § Per-session mode switching](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md); the `bash/sandbox-mode` fold owned by [`dsh-bash`](../bash/README.md)) real at EXECUTION: every call is stamped `escalation grant > session override > undefined` onto `BashExecRequest.sandboxMode`; without either, the executor's `resolve()` applies its configured default. Nothing is stamped under a non-sandboxing executor (nothing would honor it) or for an agent-less caller (no session to fold). The prompt deliberately does NOT state the mode and a switch is not narrated: a standing declaration teaches the model to refuse preemptively, while the denial marker already names the mode the command ran under exactly when the boundary is hit — behavior, not belief, carries the state.