fix: use upsert and deterministic IDs to prevent data stagnation

MCP tool_add_drawer:
- Make drawer_id content-based: hash full content instead of
  content[:100] + timestamp. Same content → same ID, eliminating
  TOCTOU race conditions
- Switch from col.add() to col.upsert() so re-filing with updated
  content updates the existing drawer

miner.add_drawer:
- Switch from collection.add() to collection.upsert() so re-mining
  a modified file updates instead of silently failing
- Remove the try/except catching 'already exists' — upsert handles
  this naturally

Findings: #11 (HIGH — add ignores updates), #6 (MEDIUM — TOCTOU),
          #13 (MEDIUM — non-deterministic IDs)

Includes test infrastructure from PR #131.
92 tests pass.

tests/test_knowledge_graph.py

@@ -6,6 +6,7 @@ timeline, stats, and edge cases (duplicate triples, ID collisions).
 """
 
 
+
 class TestEntityOperations:
     def test_add_entity(self, kg):
         eid = kg.add_entity("Alice", entity_type="person")
@@ -124,7 +125,6 @@ class TestWALMode:
         conn.close()
         assert mode == "wal"
 
-
 class TestStats:
     def test_stats_empty(self, kg):
         stats = kg.stats()