fix(agent-loop): park empty turn batches, keep max-tokens sticky, and settle headless at idle

- turn boundary now returns false for an empty admitted batch (claimed
  input removed before the wake) instead of opening a turn and spending a
  model call on nothing; the step boundary already had the symmetric guard.
- a max-token step stays sticky when steering or injected work continues
  the turn: a later completed step no longer downgrades the outcome,
  matching the TurnEndReasonMap contract.
- session/queue wire schema accepts the context placement (previously the
  zod union rejected injected-context snapshots wholesale and the client
  silently dropped the whole frame); schema tests cover all placements.
- headless runs settle at whole-agent idle instead of the first turn/end,
  honoring the one-shot idle-to-idle contract.
- flush JSDoc names the real callers (checkpoint policy, goal-session,
  teardown, self-flushing consumers); apiproxy zh README loses its stale
  duplicate history section; ACP note/README record the delivered error
  rejection and turnless-cancelled behaviors.

packages/host/apiproxy/tests/rpc-schemas.spec.ts

@@ -455,6 +455,17 @@ describe('events frame schemas', () => {
     }
   })
 
+  it('accepts every queue placement and rejects unknown placements', () => {
+    const item = (placement: string) => ({ type: 'session/queue', sessionId: 's', items: [{
+      id: 'm', placement,
+      message: { id: 'm', role: 'user', content: [], source: { kind: 'user' } },
+    }] })
+    for (const placement of ['queued', 'steering', 'context']) {
+      expect(() => muxFrameSchema.parse(item(placement))).not.toThrow()
+    }
+    expect(() => muxFrameSchema.parse(item('bogus'))).toThrow()
+  })
+
   it('rejects a queue snapshot with malformed items', () => {
     expect(() => muxFrameSchema.parse({ type: 'session/queue', sessionId: 's', items: 'x' })).toThrow()
     expect(() => muxFrameSchema.parse({ type: 'session/queue', sessionId: 's', items: [{ id: '', role: 'user', content: [], source: { kind: 'user' } }] })).toThrow()