fix(issue-18): default ack timeout T = 180 s

In the v4.5.0 review round grok read the brief and published `started`
70 s after the injection; with T = 60 the wrapper had already closed the
job as agent_blocked and refused its later PASS. T bounds delivery, not
work, so a generous default costs nothing when the agent answers fast.
Code, docs and the CLI snapshot updated together.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

tests/test_issue18_liveness_contract.py

@@ -133,7 +133,7 @@ def test_register_derives_the_idle_window_from_the_contract(tmp_path, monkeypatc
     reg = tmp_path / "jobs"
     jid = registry.register_job("contract", registry_dir=str(reg))
     rec = _job(reg, jid)
-    assert (rec["heartbeat_sec"], rec["heartbeat_misses"], rec["ack_timeout_sec"]) == (30, 4, 60)
+    assert (rec["heartbeat_sec"], rec["heartbeat_misses"], rec["ack_timeout_sec"]) == (30, 4, 180)
     assert rec["idle_timeout_sec"] == 120 == registry.liveness_window(30, 4)
     assert rec["transport"] == "herdr"