[store] SqliteEventLog — persistent event log backend

[dgenio]

Context

Split from #41. Implements the SQLite-backed event log, the most complex of the 4 store types.

Depends on #40 (store protocols).

Acceptance Criteria

• SqliteEventLog class in src/contextweaver/store/sqlite_event_log.py implementing EventLog protocol
• Supports all EventLog methods: append(), all(), query(), children(), parent(), count(), tail()
• Uses sqlite3 (stdlib) with WAL mode for concurrent read safety
• Supports :memory: mode for testing
• ContextItem stored via to_dict() / from_dict() in JSON columns
• Deterministic query ordering (ORDER BY rowid)
• Passes the same test suite as InMemoryEventLog via parameterized fixture
• Exported from src/contextweaver/store/__init__.py
• Full type hints, docstrings, from __future__ import annotations
• Module ≤300 lines

File Paths

• src/contextweaver/store/sqlite_event_log.py (new)
• src/contextweaver/store/__init__.py (edit — export)
• tests/test_store_event_log.py (edit — parameterize fixture for InMemory + SQLite)

Verification

make ci
pytest tests/test_store_event_log.py

Parent Issue

Part of #41 (SQLite-backed persistent stores).