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.

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -30,6 +30,8 @@ function snapshotOf(overrides: Partial<ConversationSnapshot> = {}): Conversation
 
 interface BenchOptions {
   planEntry?: React.ReactNode
+  /** The `plan` projection value the standard-kit useProjection serves. */
+  plan?: { active: boolean; pending: boolean }
   modelEntry?: React.ReactNode
   /** Hot text-ref lexicon (injects a minimal slash stub exposing only lexicon()). */
   lexicon?: ReadonlyMap<'/' | '@', readonly string[]>
@@ -88,7 +90,8 @@ function bench(over?: BenchOptions) {
       items: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })),
-    useProjection: (() => undefined),
+    useProjection: ((_key: string, selector?: (v: unknown) => unknown) =>
+      (selector ?? (v => v))(over?.plan)),
     useInput: bindSnapshotSelector(shell.state),
     inputActions: shell.actions,
     keyboard: shell,
@@ -230,6 +233,20 @@ describe('running and lock semantics (queue cut 1)', () => {
     const custom = bench({ placeholder: 'Custom placeholder' })
     expect(custom.textarea.placeholder).toBe('Custom placeholder')
   })
+
+  it('the plan projection swaps the placeholder while its effective target is plan mode', () => {
+    const active = bench({ plan: { active: true, pending: false } })
+    expect(active.textarea.placeholder).toBe('describe your task to generate plan')
+    // /plan just ran: pending entry already reads as the plan target.
+    const entering = bench({ plan: { active: false, pending: true } })
+    expect(entering.textarea.placeholder).toBe('describe your task to generate plan')
+    // Pending exit: target is default again.
+    const leaving = bench({ plan: { active: true, pending: true } })
+    expect(leaving.textarea.placeholder).toBe('Message the agent')
+    // Owner placeholder outranks the plan swap.
+    const custom = bench({ plan: { active: true, pending: false }, placeholder: 'Custom placeholder' })
+    expect(custom.textarea.placeholder).toBe('Custom placeholder')
+  })
 })
 
 describe('machine pending lock', () => {