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/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"}}