Add the operator kill switch, stop (A4)

`stop` could not have worked by adding the command alone. Planning
turns, milestones and review responses ran synchronously on the thread
that delivered the message, so the Matrix sync loop read nothing until
the work finished. The state store was also opened on the main thread
and used from the transport and poller threads, which SQLite refuses
with its default same-thread check; the in-memory test transport never
showed it.

A single worker thread now runs every handler and owns the store:

- The worker opens the store, so the same-thread check stays on and
  fails loudly if anything touches the store from elsewhere
- Matrix messages are handed to the worker; pollers hand over and wait,
  so their cursor never passes an unhandled comment
- A message that arrives during long work gets an immediate "busy; say
  'stop' to halt" reply and is recorded, not applied later in a state
  the operator was not answering
- Tests keep running inline through InlineWorker, which defers a job
  submitted from inside another as the real worker would queue it

`stop` sets a shared Cancel flag straight from the transport's thread.
The executor and delegation loops check it before each model call and
between tool calls; the planner checks it before each turn. Calls left
unrun get a result saying so, so the transcript still renders. A
stopped delegation is discarded like a failed one. The orchestrator
moves to AWAIT_INPUT and asks why the work was stopped. The reply is
recorded for the model, and a stopped planning turn resumes with it as
the answer. `stop` with nothing running says so and changes nothing; at
a gate it is no longer taken as rejection feedback.

Closes #40

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

nisse-dev/tests/test_agent_build.py

@@ -3,7 +3,6 @@ delegation harness and the comment poller — were fully built, fully tested,
 and reachable from nothing but their own tests. Unit tests that construct a
 component directly cannot catch that; only assembly can."""
 
-import sqlite3
 from pathlib import Path
 from typing import Any
 
@@ -163,8 +162,7 @@ def test_stop_closes_everything_and_releases_the_wait(
 
     assert agent.transport.closed
     agent.wait()  # returns immediately rather than hanging
-    with pytest.raises(sqlite3.ProgrammingError):
-        agent.store.get_state()  # the store really is closed
+    assert agent.store.closed
 
 
 def test_poller_factory_ignores_the_agents_own_comments(
@@ -268,7 +266,8 @@ def test_the_resolved_model_is_pinned_at_task_start(
     tmp_path: Path, origin: Path, monkeypatch
 ) -> None:
     agent = build_offline(tmp_path, origin, monkeypatch)
-    agent.orchestrator._begin_project("greet")
+    # The store belongs to the worker thread, so the work has to run there.
+    agent.worker.call(lambda: agent.orchestrator._begin_project("greet"))
 
     pinned = agent.orchestrator._data["models"]
     assert pinned["planner"] == {