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/planner.py

@@ -11,8 +11,9 @@ crash mid-planning leaves a truthful transcript.
 
 from typing import Any
 
+from nisse_dev.cancel import Cancel
 from nisse_dev.canonical import AssistantText, ToolCall, ToolResult
-from nisse_dev.control import AskHuman, PlanningError, SubmitPlan
+from nisse_dev.control import AskHuman, PlanningError, PlanningStopped, SubmitPlan
 from nisse_dev.llm import BudgetExhausted, LLMClient, LLMError, RoleMap
 from nisse_dev.state import StateStore
 
@@ -84,11 +85,15 @@ class LLMPlanningAgent:
         roles: RoleMap,
         *,
         system_prompt: str = SYSTEM_PROMPT,
+        cancel: Cancel | None = None,
     ) -> None:
         self._client = client
         self._store = store
         self._roles = roles
         self._system_prompt = system_prompt
+        # Checked before each turn only. A turn is one model call, and a turn
+        # that completes ends at a gate or a question, which is a halt anyway.
+        self._cancel = cancel or Cancel()
 
     def plan(
         self,
@@ -105,9 +110,11 @@ class LLMPlanningAgent:
             f"{self._system_prompt}\n\n"
             f"Current task: draft the {gate} plan for project {project!r}."
         )
+        self._check_stop()
         action, first_error = self._try_turn(model, instruction)
         if action is not None:
             return action
+        self._check_stop()
         action, second_error = self._try_turn(model, f"{instruction}\n\n{_NUDGE}")
         if action is None:
             detail = second_error or first_error
@@ -117,6 +124,10 @@ class LLMPlanningAgent:
             )
         return action
 
+    def _check_stop(self) -> None:
+        if self._cancel.requested:
+            raise PlanningStopped("stopped by the operator")
+
     def _try_turn(
         self, model: str, system: str
     ) -> tuple[SubmitPlan | AskHuman | None, str | None]: