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/ui/acp/README.md

@@ -32,11 +32,18 @@ The `initialize` handshake reports a fixed server identity (`agentInfo: { name:
 | `session/update` | `session/event` | `agent_message_chunk` (text-delta), `agent_thought_chunk` (reasoning-delta), `user_message_chunk` (load replay), `tool_call`/`tool_call_update` (the render intent — a `card`-tagged `ToolCallView`/`ToolResultView` — owned by the TOOL via `presentCall`/`presentResult`, which the bridge switches on to build the wire shape — see Tool-call presentation) |
 | `elicitation/create` | `ctx.userInteraction.ask()` | maps `ask_user_question` questions to ACP form elicitations; option descriptions are shown in enum titles, `multi_select` uses ACP array enums, optionless requests use a required `custom` field, and a non-empty custom answer overrides any selected choice |
 | `session/request_permission` | `approval/request` listener | the bridge is the [`ctx.approval`](../../approval/approval/README.md) answerer for the agents it owns: an `ask` (a hook or `tools/pre-execute` plugin) becomes an editor prompt attached to the streamed tool call, offering one-shot `allow_once`/`reject_once` options only; a foreign or call-less request delegates down the answerer chain (fail-closed `unavailable` default). See "Permission prompts" |
+| `session/set_config_option` | `setSandboxMode` / `setApprovalPolicy` | per-session knob switching over [session config options](https://agentclientprotocol.com/protocol/session-config-options) — see "Session config options" |
 
 ## Multi-session
 
 The bridge multiplexes N sessions over one connection. Live sessions are held in a `Map<sessionId, SessionRecord>` (forward) with a `WeakMap<Agent, sessionId>` reverse map so `agent/*` events — which carry only the `Agent` — demux in O(1). Every `session/event` and `agent/status` is routed strictly to its owning record, so concurrent sessions never cross-settle or interleave their `session/update` notifications. State is per session: one in-flight prompt each, `session/cancel` aborts and settles only its own agent/prompt, and disposal drains every live session in parallel to quiescence. Permission prompts follow the same ownership: the `approval/request` answerer resolves the owning session through the reverse map and prompts only there.
 
+## Session config options
+
+The bridge advertises one independent `select` per composable knob in the `session/new`/`session/load` responses — `sandbox-mode` (`read-only`/`workspace-write`/`danger-full-access`, category `mode`) iff the mounted executor confines (`ctx.get('bash')?.sandboxMode` defined), `approval-policy` (`ask`/`never`) iff the approval seam is composed — with each session's `currentValue` folded from its OWN log (`effectiveSandboxMode`/`effectiveApprovalPolicy` ?? the composition default), so `session/load` reports a resumed session's overrides with no catch-up machinery. `session/set_config_option` validates the value against the same closed vocabulary, routes to the domain's write path (`setSandboxMode`/`setApprovalPolicy` — ONE log-only event on that session's log), and returns the complete refreshed state per the spec. Anchoring honors turn-enclosure: a switch while a turn is open appends immediately (openness read from the LOG — `agent.status` stays `running` between queued turns); an idle switch is held on the session record and anchored at the next turn's `agent/prompt-submit` (inside the turn, before anything assembles, last write per knob — an idle flip-flop anchors as one event), because appending from inside a `session/event` listener would reorder events for later-registered peers. Until anchored the switch lives in bridge memory only: responses overlay it truthfully, and a crash before the next turn reverts it — `session/load` then reports the fold's truth. Design: [the sandbox RFC § Per-session mode switching](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md); protocol matrix: [acp-feature-support.md](acp-feature-support.md) § 6.
+
+Background-task isolation rides on `dsh-tool-bash`: bash task ids are global and predictable, so each task carries an opaque owner token — the owning agent's `session.header.id` — stored on the task inside the executor (`dsh-bash`'s `ownerOf(id)` seam). `bash_output`/`bash_kill` reject a task whose token differs from the caller's session token, so one session's agent can't read or kill another's task. Ownership is by session TOKEN, not `Agent` object identity — a different `Agent` object on the same session may access the task — and because the token lives on the executor's task it survives a `tool-bash` HMR reload.
+
 ## Per-session cwd
 
 Each session runs in its own workspace, recorded as the session's `SessionHeader.cwd`. On `session/new` the (absolute) request `cwd` becomes that header cwd; on `session/load` the resumed session keeps its PERSISTED header cwd and the request `cwd` must be absolute and equal to it, so the editor and bash executor agree on the workspace before an agent is constructed. A load whose persisted session has no absolute cwd is REJECTED up front via a metadata-only `list()` check, BEFORE resume constructs an agent (else bash would silently fall back to the server's launch dir, and a post-resume reject would leak the registered agent). `dsh-tool-bash` then defaults the bash workdir to the calling agent's `session.header.cwd` (an explicit model `workdir` still wins; a relative one resolves against the session cwd; with no session cwd the executor falls back to its own config / `process.cwd()`). So the server no longer has to be launched in the workspace — an editor can open any project folder, and N sessions over one connection can each target a different directory. (`additionalDirectories` is still rejected: widening the tool/filesystem scope beyond the single cwd is a separate sandbox concern.)