Implement A9 state-change logging in the state store

Retrofit for the conventions merged in #4 (issue #3):

- OpLog: JSON-lines emitter with the fixed envelope (ts, agent,
  event, task_id, cause); task_id set by the orchestrator at
  unit-of-work start
- StateStore owns emission: transition, intent begin/commit/abort,
  and schema migrations each emit their event right after the
  durable write commits; callers pass the cause
- reconcile_on_boot emits boot.reconcile with action and verdicts,
  and resolves intents with reconciliation-verdict causes
- Failed resolutions emit nothing (a line exists iff the write
  committed)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nisse-dev/tests/test_reconcile.py

@@ -18,35 +18,35 @@ def test_no_pending_intents_resumes(store: StateStore) -> None:
 
 
 def test_verified_effect_commits_intent_and_resumes(store: StateStore) -> None:
-    store.begin_intent("push_branch", {"branch": "m2"})
+    store.begin_intent("push_branch", {"branch": "m2"}, cause="test")
     report = reconcile_on_boot(store, {"push_branch": lambda intent: True})
     assert report.action == "resume"
     assert store.pending_intents() == []
 
 
 def test_verified_no_effect_aborts_intent_and_resumes(store: StateStore) -> None:
-    store.begin_intent("push_branch", {"branch": "m2"})
+    store.begin_intent("push_branch", {"branch": "m2"}, cause="test")
     report = reconcile_on_boot(store, {"push_branch": lambda intent: False})
     assert report.action == "resume"
     assert store.pending_intents() == []
 
 
 def test_unverifiable_intent_gates_to_await_input(store: StateStore) -> None:
-    store.begin_intent("push_branch", {"branch": "m2"})
+    store.begin_intent("push_branch", {"branch": "m2"}, cause="test")
     report = reconcile_on_boot(store, {"push_branch": lambda intent: None})
     assert report.action == "await_input"
     assert [i.kind for i in store.pending_intents()] == ["push_branch"]
 
 
 def test_unknown_intent_kind_gates_to_await_input(store: StateStore) -> None:
-    store.begin_intent("mystery", {})
+    store.begin_intent("mystery", {}, cause="test")
     report = reconcile_on_boot(store, {})
     assert report.action == "await_input"
     assert "no reconciler registered" in report.notes[0]
 
 
 def test_reconciler_receives_the_intent(store: StateStore) -> None:
-    store.begin_intent("push_branch", {"branch": "m2"})
+    store.begin_intent("push_branch", {"branch": "m2"}, cause="test")
     seen: list[Intent] = []
 
     def check(intent: Intent) -> bool:
@@ -58,8 +58,8 @@ def test_reconciler_receives_the_intent(store: StateStore) -> None:
 
 
 def test_mixed_verdicts_still_gate(store: StateStore) -> None:
-    store.begin_intent("push_branch", {})
-    store.begin_intent("open_pr", {})
+    store.begin_intent("push_branch", {}, cause="test")
+    store.begin_intent("open_pr", {}, cause="test")
     report = reconcile_on_boot(
         store,
         {"push_branch": lambda intent: True, "open_pr": lambda intent: None},