feat(issue-5): shadow watcher that logs would-complete and cannot write

policy.decide is a pure function so the phase-0 scenario fixtures can be fed to
it verbatim. Those seven decisions survived three-reviewer scrutiny, so they are
the contract rather than a fresh set of rules, and the test asserts against
expected_policy.decision directly.

Order inside the policy matters. Transport doubt and staleness invalidate the
evidence itself, so they are answered before anything is inferred from it; a
blocked agent is waiting for a human and can never be completed; attribution
including the execution anchor comes before quiescence, because idle is not
evidence of finishing when a session is reused; and a report must be finalized,
never merely paused, since a debounce is a candidate filter and not an authority.

The shadow runner is never handed the engine, so it has no route to a write.
That is the structural half of P2A-03. The measured half hashes every file under
the registry directory around a full run and asserts it is unchanged, which is a
stronger claim than checking a status field.

Adds the five scenarios P2A-04 requires: a raced double dispatch, a reused
attempt with no anchor, a mid-write report, an unknown status, and a pane
restarted mid-flight. They pass phase 0's own fixture validator, which is what
keeps them honest. The phase-0 test that pinned exactly seven scenarios now
requires the seven to still exist rather than forbidding growth.

Also pins the two phase 1 handoffs as dormant: nothing here spawns a daemon, so
preexec_fn stays unreachable, and nothing calls a frozen shell helper.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UrDNJkHmm18uu7n7JwV5Qd

.agents/skills/lib_py/loop/shadow.py

@@ -0,0 +1,51 @@
+"""Shadow runner: decides what it *would* commit, and cannot commit it.
+
+P2A-03 requires that this phase never writes registry state. That is enforced
+structurally rather than promised: ShadowRunner is never given the transition
+engine, so it has no means to write. The gate goes further and hashes the whole
+registry directory around a run, which is a stronger claim than checking that a
+status field did not change.
+"""
+from __future__ import annotations
+
+import logging
+from typing import Any, Callable, Dict, Optional
+
+from lib_py.loop import policy, store
+
+logger = logging.getLogger("loop.shadow")
+
+
+class ShadowRunner(object):
+    """Runs observation -> decision -> log. Holds no writer of any kind."""
+
+    def __init__(self, workspace=None, decide: Callable = policy.decide, sink=None):
+        self.workspace = workspace
+        self._decide = decide
+        self._sink = sink if sink is not None else self._default_sink
+
+    @staticmethod
+    def _default_sink(job_id, entry, workspace=None):
+        store.append_decision(job_id, entry, workspace=workspace)
+
+    def evaluate(self, scenario: Dict[str, Any]) -> Dict[str, Any]:
+        """Decide for one composite scenario and record the would-be outcome."""
+        job_id = (scenario.get("dispatch_context") or {}).get("job_id") or \
+            scenario.get("scenario_id") or "unknown"
+        decision = self._decide(scenario)
+        entry = {
+            "job_id": job_id,
+            "scenario_id": scenario.get("scenario_id"),
+            "would_complete": decision.would_complete,
+            "shadow": True,
+        }
+        entry.update(decision.as_dict())
+        self._sink(job_id, entry, workspace=self.workspace)
+        if decision.would_complete:
+            logger.info("shadow: WOULD-COMPLETE job %s (%s)", job_id, decision.reason)
+        else:
+            logger.debug("shadow: hold job %s (%s)", job_id, decision.reason)
+        return entry
+
+    def evaluate_all(self, scenarios) -> list:
+        return [self.evaluate(s) for s in scenarios]