⚡ Bolt: optimize SessionManager database operations

[charles-forsyth]

💡 What:

• Optimized SessionManager.list_sessions by using a LEFT JOIN to fetch parent session status and PID in a single query, eliminating the N+1 query problem.
• Batched UPDATE statements for 'crashed' sessions into a single query using WHERE id IN (...).
• Added database indexes on interaction_id, parent_id, and updated_at to improve lookup and sorting performance.
• Refactored append_to_result to use a single SQL UPDATE with concatenation, eliminating a redundant SELECT.
• Implemented a thread-safe class-level cache to skip redundant _init_db (schema check and migration) calls within the same process.

🎯 Why:

• The list_sessions method was performing an additional SELECT query for every running child session to check its parent's status, leading to $O(N)$ database round-trips.
• Frequent schema checks in every SessionManager instantiation were adding unnecessary overhead, especially in recursive research mode where many agents are spawned.
• Large session histories would eventually slow down due to missing indexes on frequently queried columns.

📊 Impact:

• Reduces database queries in list_sessions from $2N+1$ to constant $2$ queries in the worst case (1 SELECT, 1 batched UPDATE).
• Measured a ~9x speedup in list_sessions for 100 sessions (from 0.086s to 0.010s).
• Improves responsiveness of the list command and overall system efficiency.

🔬 Measurement:

• Verified with a performance script that counts sqlite3.Connection.execute calls.
• Confirmed that all existing 30 tests pass (uv run pytest).

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PR created automatically by Jules for task 5037856749770711362 started by @charles-forsyth

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