⚡ Bolt: Optimize session archiving cleanup

[google-labs-jules]

💡 What: Replaced O(N^2) array .find() in chunked loops with a pre-built O(N) Map lookup and added missing try...catch block to sequential operations on same entity.
🎯 Why: To fix performance degradation scaling with total sessions when archiving medium-priority sessions, which causes redundant array scans, and ensure we do not delete original file if archiving fails.
📊 Impact: Changes O(N^2) array lookup overhead inside concurrent Promise mapping into O(N) linear time for building Map and O(1) for getting session, making cleanup performant for large session counts.
🔬 Measurement: Run bun test ./tests/unit/core/session/manager-archive.test.ts and bun run test:full to verify safety.

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PR created automatically by Jules for task 16105295175900968238 started by @YoungSx

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