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>

tests/integration/test_crash_durability.py

@@ -18,8 +18,8 @@ import sys
 from nisse_dev.state import StateStore
 
 store = StateStore(sys.argv[1])
-store.transition("EXECUTING", project="demo", data={"milestone": 2})
-store.begin_intent("push_branch", {"branch": "milestone-2"})
+store.transition("EXECUTING", cause="test", project="demo", data={"milestone": 2})
+store.begin_intent("push_branch", {"branch": "milestone-2"}, cause="test")
 os._exit(1)  # die without closing anything, as a power cut would
 """