fix: make knowledge_graph.py SQLite connections exception-safe

[adv3nt3]

Summary

• Wrap all 8 methods in KnowledgeGraph that open SQLite connections with contextlib.closing() to guarantee conn.close() is called even if execute()/commit()/fetchall() raises. Previously, connections would leak on exception.
• Add transaction context manager (with conn:) to write methods (add_entity, add_triple, invalidate) for auto-commit on success and auto-rollback on exception.
• Read methods (query_entity, query_relationship, timeline, stats) use closing() only.
• _init_db keeps manual commit() since executescript has its own transaction semantics.
• Simplifies the add_triple early-return path — no more redundant manual conn.close().

Test plan

• ruff check mempalace/knowledge_graph.py passes clean
• ruff format --check mempalace/knowledge_graph.py already formatted
• python3 -m py_compile mempalace/knowledge_graph.py compiles OK
• Pattern validated against CPython sqlite3 docs via Context7
• Pyright reports 0 new diagnostics (10 pre-existing Optional type warnings unchanged)

[adv3nt3]

The CI failure is not from this PR — both failing tests are in test_dialect.py, unrelated to knowledge_graph.py:

1. TestCompressionStats::test_stats — expects stats["ratio"] key that was removed when PR fix: honest AAAK stats — word-based token estimator, lossy labels #147 changed the compression stats API
2. TestCompressionStats::test_count_tokens — expects the old len(text)//3 heuristic, but PR fix: honest AAAK stats — word-based token estimator, lossy labels #147 switched to word-based token counting

This is a pre-existing test-vs-code mismatch on main. PR #150 ("test(dialect): update assertions for new honest-stats API") is the fix for exactly these failures.

All 97 other tests pass, including the knowledge graph tests that exercise the code this PR changes.

[adv3nt3]

@bensig @milla-jovovich Closing this, the upstream changes to knowledge_graph.py solve both problems this PR addressed:

1. Connection leaks — now handled by a persistent self._connection with an explicit close() method, instead of per-call connections that could leak on exception
2. Transaction safety — with conn: blocks are now used for write operations, providing auto-commit/rollback

The persistent connection + sqlite3.Row approach is cleaner than the contextlib.closing() pattern this PR used. No need to merge this anymore.