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/acp-feature-support.md

@@ -10,7 +10,7 @@ Legend: ✅ supported · ⚠️ partial / fallback · ❌ not yet · — n/a. Th
 
 ## At a glance
 
-The bridge implements the **core prompt-turn loop** for N concurrent sessions: initialize, session new/load, prompt, cancel, streamed assistant/thought chunks, tool-call rendering (including Zed terminal cards), and resumable session replay. The largest **unbuilt** areas are **MCP passthrough**, **session modes / config options / model selection**, **slash commands**, and **agent plans** — all of which both reference adapters ship — plus the client **filesystem** and **terminal** method families (which the adapters mostly do NOT drive either — see rows 43-49). See [Gap summary](#gap-summary).
+The bridge implements the **core prompt-turn loop** for N concurrent sessions: initialize, session new/load, prompt, cancel, streamed assistant/thought chunks, tool-call rendering (including Zed terminal cards), and resumable session replay. The largest **unbuilt** areas are the **permission gate** (`session/request_permission`), **MCP passthrough**, **session modes / config options / model selection**, **slash commands**, and **agent plans** — all of which both reference adapters ship — plus the client **filesystem** and **terminal** method families (which the adapters mostly do NOT drive either — see rows 43-49). See [Gap summary](#gap-summary).
 
 ## 1. Agent methods (client → agent)
 
@@ -26,7 +26,7 @@ The bridge implements the **core prompt-turn loop** for N concurrent sessions: i
 | `session/prompt` | S | ✅ | ✅ | ✅ | Maps to `agent.send`; one in-flight prompt per session; settles on the owning turn's end. |
 | `session/cancel` | S | ✅ | ✅ | ✅ | Queue-aware `agent.cancel`; settles the in-flight prompt `cancelled`, scoped to the one session. |
 | `session/set_mode` | S | ❌ | ✅ | ✅ | Session modes deliberately skipped: config options are the spec's replacement (modes are slated for removal in ACP v2), and one mode list cannot carry the two orthogonal knobs (see [§6](#6-session-modes--config-options--models)). |
-| `session/set_config_option` | S | ❌ | ✅ | ✅ | Config options not modeled yet — the sandbox RFC's per-session mode switching stages them ([sandbox RFC](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md)). |
+| `session/set_config_option` | S | ✅ | ✅ | ✅ | Two capability-gated selects — `sandbox-mode` (confining executor mounted) and `approval-policy` (approval seam composed); values validated against the domain vocabularies, one log-only event per switch on the session's own log, complete refreshed state in the response ([sandbox RFC § Per-session mode switching](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md)). |
 | model selection | S | ❌ | ✅ | ✅ | No distinct stable `session/set_model` — model is the `model`-category `session/set_config_option`. The bridge fixes the model per-bridge via config; no runtime switch. Codex still uses the legacy `unstable_setSessionModel` ext method. |
 | `session/list` | S | ❌ | ✅ | ✅ | Gated by `sessionCapabilities.list`. The harness HAS `sessionPersistence.list()` (used internally for load-cwd validation) but does not expose it over ACP. |
 | `session/delete` | S | ❌ | ✅ | ✅ | Gated by `sessionCapabilities.delete`. |
@@ -86,7 +86,7 @@ These are capabilities the bridge would *drive* on the editor. The harness runs
 | `plan` | S | ❌ | ✅ | ✅ | No agent plan emitted. Both adapters emit real plan entries (Codex's `CodexEventHandler.updatePlan` maps `turn/plan/updated` → `{ sessionUpdate: 'plan', entries }`). |
 | `available_commands_update` | S | ❌ | ✅ | ✅ | No slash commands advertised. |
 | `current_mode_update` | S | ❌ | ✅ | ✅ | No session modes. |
-| `config_option_update` | S | ❌ | ✅ | ✅ | No config options yet. |
+| `config_option_update` | S | ❌ | ✅ | ✅ | Config options exist (advertised in `session/new`/`session/load`, switched via `session/set_config_option`), but the bridge never pushes agent-initiated changes — an operator default drift is narrated to the MODEL, not echoed to the editor. Future work in the [sandbox RFC § Per-session mode switching](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md). |
 | `usage_update` | S | ❌ | ✅ | ✅ | Token/cost reporting not surfaced (the harness records token usage internally on `assistant/message`). |
 | `session_info_update` | S | ❌ | ⚠️ | ⚠️ | Session title/metadata not pushed. |
 
@@ -111,7 +111,7 @@ Tool-call presentation is **owned by each tool** (`presentCall` / `presentResult
 
 ## 6. Session modes / config options / models
 
-Session modes and config options are not modeled yet: the sandbox RFC's per-session mode switching ([sandbox RFC](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md)) stages config options as the surface (modes are slated for removal in ACP v2, and one mode list cannot carry two orthogonal knobs). Runtime model selection is also not modeled — the harness fixes the model per-bridge via `AcpConfig.model` (both reference adapters ship a model selector).
+Config options ✅ (the [sandbox RFC § Per-session mode switching](../../../docs/rfc/proposed/feature/2026-07-06-sandbox.md)): the bridge advertises one independent `select` per composable knob — `sandbox-mode` iff the mounted executor confines, `approval-policy` iff the approval seam is composed — with per-session current values folded from each session's own log, and honors `session/set_config_option` end to end (idle switches anchor at the next turn under the turn-enclosure contract). Session MODES stay deliberately unmodeled: config options are the spec's replacement (modes are slated for removal in ACP v2), and one mode list cannot carry two orthogonal knobs. Runtime model selection is still not modeled — the harness fixes the model per-bridge via `AcpConfig.model` (both reference adapters ship a model selector).
 
 ## 7. Content blocks
 
@@ -141,7 +141,7 @@ The bridge rejects unsupported prompt blocks rather than silently dropping them
 Ranked by how commonly the reference adapters ship them and how much UX they unlock:
 
 1. **Session lifecycle** — `session/list` + `session/delete` (the persistence layer already lists), then `session/resume` / `session/close`.
-2. **Modes / config options / model selection** — the permission round-trip landed with the approval seam; the config surface (`sandbox-mode`/`approval-policy` options) is the sandbox RFC's staged config phase.
+2. **Modes / config options / model selection** — the permission round-trip landed with the approval seam; the config surface (`sandbox_mode`/`approval_policy` options) is the sandbox RFC's config phase.
 3. **Agent plan** (`sessionUpdate: 'plan'`) — surface the loop's plan as structured entries.
 4. **Slash commands** (`available_commands_update`).
 5. **MCP passthrough** (`mcpServers` on `session/new` + `mcpCapabilities`).