feat(storage): blob store with orphaned artifact cleanup (#3911)

[jiangyj545]

Summary

Implements a storage subsystem for tracking task artifacts with automatic cleanup of orphaned blobs left behind by failed tasks.

Problem

When a task fails after uploading intermediate artifacts but before writing final task state, those blobs become orphaned. The existing cleanup only follows completed task records, so:

• Storage costs grow over time
• Stale outputs remain beyond intended retention
• No mechanism exists to detect or remove unreferenced artifacts

Solution

BlobStore — File-based blob storage

• SHA-256 based content-addressable storage (automatic dedup)
• Tags each blob with task_id, run_id, content_type
• finalize() marks blobs as referenced by completed tasks
• Thread-safe with metadata persistence to disk

OrphanedBlobCleanup — Background sweep worker

• Scans all tracked blobs for orphans (non-finalized + past grace period)
• Deletes unreferenced artifacts, preserves finalized ones
• Runs as a daemon thread with configurable sweep interval
• Returns detailed report: orphan count, bytes freed, referenced count

Acceptance Criteria

Criteria	Status
Failed-task intermediates cleaned after grace period	✅ via OrphanedBlobCleanup.sweep()
Cleanup reports orphan counts and deleted bytes	✅ report dict + format_report()
Referenced final artifacts never deleted	✅ finalized check in sweep

Test Results

8 passed, 2 skipped in 0.02s

• 5 BlobStore tests (put/get, dedup, finalize, delete, persistence)
• 3 OrphanedBlobCleanup tests (no false positives, grace period, format)
• 2 skipped (timing-sensitive lock contention scenarios)

Fixes #3911