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/delegation.py

@@ -28,6 +28,7 @@ from collections.abc import Callable
 from dataclasses import dataclass, field
 from typing import Any, Literal
 
+from nisse_dev.cancel import Cancel, skip_calls
 from nisse_dev.canonical import (
     AssistantText,
     DelegationCompleted,
@@ -129,7 +130,7 @@ class Handback:
 
 @dataclass
 class DelegationResult:
-    status: Literal["handback", "capped", "failed", "interrupted"]
+    status: Literal["handback", "capped", "failed", "interrupted", "stopped"]
     note: str
     handback: Handback | None = None
     branch: str | None = None
@@ -150,9 +151,9 @@ class DelegationResult:
 
     @property
     def usable(self) -> bool:
-        """Only a successful handback may be built upon. A cap breach or a
-        failure leaves a half-edited tree, which A13 says must gate rather
-        than be resumed."""
+        """Only a successful handback may be built upon. A cap breach, a
+        failure or an operator's stop leaves a half-edited tree, which A13
+        says must gate rather than be resumed."""
         return self.status == "handback" and self.handback is not None
 
 
@@ -168,6 +169,7 @@ class Delegation:
         backing_model: str = "",
         caps: Caps | None = None,
         now: Callable[[], float] = time.monotonic,
+        cancel: Cancel | None = None,
     ) -> None:
         self._client = client
         self._store = store
@@ -177,6 +179,10 @@ class Delegation:
         self._backing_model = backing_model
         self._caps = caps or Caps()
         self._now = now
+        # A stopped delegation is discarded like a failed one, not preserved
+        # for resuming as an interrupted one is: the operator stopped it for
+        # a reason, and the next step is to talk about that reason.
+        self._cancel = cancel or Cancel()
 
     def run(
         self, *, brief: str, branch: str, role: str = "delegate"
@@ -316,6 +322,14 @@ class Delegation:
         spent = 0
 
         for turn in range(self._caps.turns):
+            if self._cancel.requested:
+                return DelegationResult(
+                    "stopped",
+                    f"stopped by the operator before turn {turn + 1}",
+                    branch=branch,
+                    artifacts=sorted(touched),
+                    tokens=spent,
+                )
             elapsed = self._now() - started_at
             if elapsed >= self._caps.wall_clock_s:
                 return DelegationResult(
@@ -386,7 +400,16 @@ class Delegation:
                     tokens=spent,
                 )
 
-            for call in calls:
+            for index, call in enumerate(calls):
+                if self._cancel.requested:
+                    skip_calls(self._store, calls[index:])
+                    return DelegationResult(
+                        "stopped",
+                        f"stopped by the operator during turn {turn + 1}",
+                        branch=branch,
+                        artifacts=sorted(touched),
+                        tokens=spent,
+                    )
                 if call.tool == "hand_back":
                     handback = _parse_handback(call.arguments)
                     if handback is not None and handback.succeeded and not did_work: