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/core/agent-loop/tests/loop.spec.ts

@@ -967,7 +967,9 @@ describe('agent loop', () => {
         source: { kind: 'plugin', plugin: 'max-tokens-test' },
       },
     ])
-    expect(reasons).toEqual([{ kind: 'completed' }])
+    // A max-token step is sticky: the later completed step must not
+    // downgrade the turn outcome.
+    expect(reasons).toEqual([{ kind: 'max-tokens' }])
   })
 
   it('a completed step after no max-tokens keeps the turn completed (max-tokens does not leak across turns)', async () => {