feat(web): replace the plan select with a status chip and placeholder swap

Plan mode is entered through /plan only — the select control is retired.
The conversation.input.plan seat (now right of the access-mode control)
renders a read-only Plan chip while the projection's effective target is
plan mode; its hover x executes /plan off, and the chip follows the folded
target (appears on /plan immediately, disappears on /plan off) with frames
correcting either way. While plan mode is targeted the composer textarea's
placeholder switches to the plan-task wording — InputBar reads the same
projection through the standard-kit useProjection (the TodoDock posture:
a type-only key merge, no domain service edge), and owner placeholders
still win.

scripts/verify-package-readme-model-experience.ts

@@ -58,7 +58,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/client/ui-slash': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/ui-command': { kind: 'indirect', reason: 'The dispatch paths trigger the host command.execute RPC; each command handler\'s host package owns any model-visible effect.' },
   'packages/client/ui-model': { kind: 'indirect', reason: 'Selection routes session.selectModel; the host snapshots the target at the next prompt-assembly boundary and owns the model-visible effect.' },
-  'packages/client/ui-plan': { kind: 'indirect', reason: 'The control dispatches /plan command lines; dsh-plan-mode owns the model-visible policy, exit tool, and logged state.' },
+  'packages/client/ui-plan': { kind: 'indirect', reason: 'The chip dispatches /plan off; dsh-plan-mode owns the model-visible policy, exit tool, and logged state.' },
   'packages/client/ui-question': { kind: 'indirect', reason: 'The package mounts dsh-tool-ask-user; that tool owns the model-visible schema and answer rendering.' },
   'packages/client/ui-trajectory': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/ui-workspace': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },