Add durable state store and boot-time reconciliation (A2/A3)

Build-order step 1: the critical path everything else sits on.

- StateStore: SQLite in WAL mode with synchronous=FULL, so every
  state transition and intent-log write is committed and fsynced
  before returning; a power cut cannot tear state
- Durable intent log: begin_intent records a unit of work before it
  starts; commit/abort resolve it; pending intents survive restart
- schema_version stamped from the first commit, with a sequential
  migration hook; a newer state file than the code supports is
  refused rather than misread
- reconcile_on_boot (A3 first cut): pending intents are checked
  against external truth by registered reconcilers (observable
  checks, never model self-assessment); verified effects are
  committed, verified no-ops aborted, anything unverifiable gates
  to AWAIT_INPUT — never blind-resume
- Lives in nisse_dev per spec 0.1: the library is extracted after
  the dev agent works, not designed up front
- Integration test kills a subprocess mid-work with os._exit and
  verifies state integrity and intent visibility on reopen

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

tests/integration/test_crash_durability.py

@@ -0,0 +1,47 @@
+"""Integration: state written before an abrupt process death must be intact
+and the in-flight intent visible on the next boot (spec A2/A3)."""
+
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+from nisse_dev.reconcile import reconcile_on_boot
+from nisse_dev.state import StateStore
+
+pytestmark = pytest.mark.integration
+
+CRASH_SCRIPT = """
+import os
+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"})
+os._exit(1)  # die without closing anything, as a power cut would
+"""
+
+
+def test_state_survives_process_death(tmp_path: Path) -> None:
+    db = tmp_path / "state.db"
+    result = subprocess.run(
+        [sys.executable, "-c", CRASH_SCRIPT, str(db)],
+        capture_output=True,
+        text=True,
+        timeout=30,
+        check=False,
+    )
+    assert result.returncode == 1, result.stderr
+
+    with StateStore(db) as store:
+        state = store.get_state()
+        assert state is not None
+        assert state.state == "EXECUTING"
+        assert state.data == {"milestone": 2}
+
+        report = reconcile_on_boot(store, {"push_branch": lambda intent: None})
+        assert report.action == "await_input"
+        [pending] = store.pending_intents()
+        assert pending.payload == {"branch": "milestone-2"}