feat(mode): exit_plan_mode + the ACP session-mode picker + scriptable review answers

Plan mode's stage 2 (RFC 2026-07-07-plan-mode). The exit tool: one
required plan argument (the durable log artifact), execute re-checks the
folded mode, then conducts the review over the user-interaction seam —
one single-select question (Approve / Keep planning) with free text open
— so an approval appends mode/set back to default in-turn and every
other outcome (keep-planning feedback verbatim, aborted, no provider)
returns the corrective isError with the mode unchanged. presentCall is a
generic card titled by the plan's first heading carrying the plan
markdown; over ACP the review rides the ask_user elicitation flow, in
the terminal the stdio prompt queue — no approval-seam dependency.

The ACP bridge maps the picker 1:1 onto ctx.modes (opportunistic, a
type-only peer edge): session/new + session/load advertise
availableModes/currentModeId, session/set_mode validates through set()
and echoes an optimistic current_mode_update (the pending mode IS the
selection; the logged mode/set lands at the boundary and, matching, is
not re-sent), and a session/event listener re-notifies on each logged
flip that differs from the last sent — the tool-driven exit updates the
picker. The feature matrix rows move from 'not modeled' to the
picker-to-modes / knobs-to-config-options division, with the ACP v2
removal direction recorded as a mechanical-migration risk.

The snapshot harness gains the setMode/setModeExpectError ops and a
scripted elicitationAnswers FIFO (cancel on exhaustion; a stray choice
string reaches the agent verbatim as a non-consenting custom answer, so
a scenario bug fails safe). The suite factory's header-pin requirement
now applies only to model-turn scenarios — a protocol-only suite has no
header content to anchor. examples/plan-acp-agent is the live
composition; its keyless modes-advertise scenario pins the wire surface
(advertisement, both set_mode round-trips, unknown-id rejection). The
recorded plan-mode approve/reject arc awaits a with-key recording
session; its texts are pinned at the unit tier meanwhile.

examples/AGENTS.md ceiling 653 → 680: the new example's required smoke
row does not fit the old budget.

examples/plan-acp-agent/tests/acp.snapshot.ts

@@ -0,0 +1,38 @@
+import { dirname, join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { defineAcpSnapshotSuite, type Scenario } from '@deepseek-ai/dsh-acp-snapshot'
+
+/**
+ * Snapshot suite for the plan-mode composition (`../cordis.yml`, swapped to
+ * the sibling `cordis.snapshot.yml` replay overlay by the bin under
+ * `DSH_SNAPSHOT=replay`).
+ *
+ * Deliberately ABSENT (pending a with-key recording session — the plan-mode
+ * RFC's recorded-scenario section): the two model-turn scenarios driving the
+ * full arc (setMode → explore → denied write → exit_plan_mode → a scripted
+ * `elicitationAnswers` approve, and the keep-planning sibling). Their deny and
+ * keep-planning texts are meanwhile pinned at the unit tier
+ * (packages/mode/mode/tests) and the ACP mode round-trip in the bridge's
+ * protocol tests (packages/ui/acp/tests/modes.spec.ts).
+ */
+const SCENARIOS: Scenario[] = [
+  // Protocol-only (keyless, authored): the session-mode surface this
+  // composition adds. No model turn runs, so no header pin
+  // is needed here — the pinning scenario arrives with the recorded plan-mode
+  // arc. Composition-wise it adds — availableModes/currentModeId advertised on
+  // session/new, the optimistic current_mode_update a session/set_mode
+  // answers with, and the loud rejection of an unknown mode id — as committed
+  // wire bytes. No model turn, so it replays keyless.
+  { name: 'modes-advertise', hasModelTurn: false, recorded: false },
+]
+
+defineAcpSnapshotSuite({
+  agent: {
+    binScript: fileURLToPath(new URL('../../../packages/ui/acp-agent/src/bin.ts', import.meta.url)),
+    configPath: fileURLToPath(new URL('../cordis.yml', import.meta.url)),
+    tsconfigPath: fileURLToPath(new URL('../../../tsconfig.json', import.meta.url)),
+  },
+  snapshotsDir: join(dirname(fileURLToPath(import.meta.url)), 'snapshots'),
+  scenarios: SCENARIOS,
+  mode: process.env.DSH_SNAPSHOT === 'record' ? 'record' : 'replay',
+})

examples/plan-acp-agent/tests/snapshots/modes-advertise/input.json

@@ -0,0 +1,22 @@
+{
+  "steps": [
+    {
+      "op": "initialize"
+    },
+    {
+      "op": "newSession"
+    },
+    {
+      "op": "setMode",
+      "modeId": "plan"
+    },
+    {
+      "op": "setMode",
+      "modeId": "default"
+    },
+    {
+      "op": "setModeExpectError",
+      "modeId": "yolo"
+    }
+  ]
+}

examples/plan-acp-agent/tests/snapshots/modes-advertise/session.jsonl

@@ -0,0 +1 @@
+{"type":"session","version":0,"id":"00000000-0000-0000-0000-000000000000","createdAt":0}

examples/plan-acp-agent/tests/snapshots/modes-advertise/stdout.golden.jsonl

@@ -0,0 +1,7 @@
+{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":1,"agentInfo":{"name":"deepseek-harness-acp","version":"0.0.1"},"agentCapabilities":{"loadSession":true,"promptCapabilities":{"image":false,"audio":false,"embeddedContext":false}},"authMethods":[]}}
+{"jsonrpc":"2.0","id":2,"result":{"sessionId":"{{sessionId}}","modes":{"availableModes":[{"id":"default","name":"default"},{"id":"plan","name":"plan"}],"currentModeId":"default"}}}
+{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"current_mode_update","currentModeId":"plan"}}}
+{"jsonrpc":"2.0","id":3,"result":{}}
+{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"{{sessionId}}","update":{"sessionUpdate":"current_mode_update","currentModeId":"default"}}}
+{"jsonrpc":"2.0","id":4,"result":{}}
+{"jsonrpc":"2.0","id":5,"error":{"code":-32602,"message":"Invalid params: unknown mode \"yolo\" — available modes: default, plan"}}