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/executor.py

@@ -14,6 +14,7 @@ from collections.abc import Callable
 from dataclasses import dataclass
 from typing import Any, Literal
 
+from nisse_dev.cancel import Cancel, skip_calls
 from nisse_dev.canonical import ToolCall, ToolResult
 from nisse_dev.delegation import Delegation
 from nisse_dev.expert import ASK_EXPERT_TOOL, BudgetExhausted, ExpertDesk
@@ -45,7 +46,7 @@ _NUDGE = (
 
 @dataclass(frozen=True)
 class ExecutorOutcome:
-    status: Literal["completed", "capped", "escalate"]
+    status: Literal["completed", "capped", "escalate", "stopped"]
     note: str
 
 
@@ -62,6 +63,7 @@ class LLMExecutor:
         expert: ExpertDesk | None = None,
         delegate: Delegation | None = None,
         notify: Callable[[str], None] | None = None,
+        cancel: Cancel | None = None,
     ) -> None:
         self._client = client
         self._store = store
@@ -75,6 +77,7 @@ class LLMExecutor:
         # without this the agent is silent for the whole milestone and a
         # working run looks exactly like a hung one.
         self._notify = notify
+        self._cancel = cancel or Cancel()
 
     def _report(self, turn: int, calls: list[ToolCall]) -> None:
         if self._notify is None:
@@ -113,6 +116,8 @@ class LLMExecutor:
         )
         if result.usable:
             return None
+        if result.status == "stopped":
+            return ExecutorOutcome("stopped", f"during a delegation: {result.note}")
         # A13: a cap breach or failure leaves a half-edited tree, so the local
         # loop must not resume on top of it.
         return ExecutorOutcome(
@@ -151,6 +156,10 @@ class LLMExecutor:
         challenged = False
 
         for turn in range(self._max_turns):
+            # A4: `stop` takes effect at the next tool boundary. The start of
+            # a turn is one, and so is the gap between two tool calls below.
+            if self._cancel.requested:
+                return ExecutorOutcome("stopped", f"before turn {turn + 1}")
             try:
                 produced = self._client.complete(
                     self._store.read_events(),
@@ -184,7 +193,10 @@ class LLMExecutor:
                 )
 
             self._report(turn, calls)
-            for call in calls:
+            for index, call in enumerate(calls):
+                if self._cancel.requested:
+                    skip_calls(self._store, calls[index:])
+                    return ExecutorOutcome("stopped", f"during turn {turn + 1}")
                 if call.tool in MUTATING_TOOLS:
                     did_work = True
                 if call.tool == "ask_expert":