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.

packages/ui/acp/tests/harness.ts

@@ -24,6 +24,7 @@ import * as FsPolicy from '@deepseek-ai/dsh-fs-policy'
 import * as ToolBash from '@deepseek-ai/dsh-tool-bash'
 import * as ToolFs from '@deepseek-ai/dsh-tool-fs'
 import * as ToolTodo from '@deepseek-ai/dsh-tool-todo'
+import ModesService from '@deepseek-ai/dsh-mode'
 import {
   ClientSideConnection,
   ndJsonStream,
@@ -180,6 +181,8 @@ export async function makeBridgeHarness(options: {
    * tool + the bridge's own todo/write→plan mapping, not a stand-in.
    */
   withTodo?: boolean
+  /** Plug the REAL `dsh-mode` plugin so a test can drive the session-mode picker. */
+  withModes?: boolean
   /**
    * Plug the REAL filesystem stack (`dsh-fs-local` + `dsh-fs-policy` +
    * `dsh-tool-fs`) so a test can drive `read`/`write`/`edit` through the bridge
@@ -211,6 +214,9 @@ export async function makeBridgeHarness(options: {
   if (options.withTodo) {
     await ctx.plugin(ToolTodo)
   }
+  if (options.withModes) {
+    await ctx.plugin(ModesService)
+  }
   if (options.withFs) {
     await ctx.plugin(LocalFileSystem, { cwd: options.fsCwd ?? options.storageDir })
     await ctx.plugin(FsPolicy)