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/src/nisse_dev/cancel.py

@@ -0,0 +1,38 @@
+"""The operator's kill switch, as the working loops see it (spec A4).
+
+`stop` has to reach work that is already running, from a thread that is not
+running it. So it is a flag, set by whoever hears the command and checked by
+the loops at each tool boundary. Setting it touches nothing durable: the
+record of a stop is written by the worker when the stopped work returns.
+"""
+
+import threading
+
+from nisse_dev.canonical import ToolCall, ToolResult
+from nisse_dev.state import StateStore
+
+STOPPED_RESULT = "not run: the operator stopped the agent"
+
+
+class Cancel:
+    def __init__(self) -> None:
+        self._event = threading.Event()
+
+    def set(self) -> None:
+        self._event.set()
+
+    def clear(self) -> None:
+        self._event.clear()
+
+    @property
+    def requested(self) -> bool:
+        return self._event.is_set()
+
+
+def skip_calls(store: StateStore, calls: list[ToolCall]) -> None:
+    """Record the calls a stop left unrun. Every tool call needs a result for
+    the transcript to render again, and the result says what happened."""
+    for call in calls:
+        store.append_event(
+            ToolResult(call_id=call.call_id, result=STOPPED_RESULT, success=False)
+        )