#175 fix: send_files collision detection + stale task cleanup

[Anonymous]

Originally created by: kumaakh

Summary

• #70 fix(T2.1): Pre-flight collision detection in send_files — collects basenames of all local_paths and returns an error listing conflicting paths before any SFTP transfer begins; adds 4 unit tests
• #8 fix(T2.2): Stale task directory cleanup with configurable retention — cleanupStaleTasks() runs at server startup, retains completed tasks 1h (FLEET_TASK_RETENTION_HOURS_SUCCESS) and failed tasks 7 days (FLEET_TASK_RETENTION_HOURS), skips running tasks; adds 10 unit tests

Test plan

• [ ] CI green (809 tests passing locally)
• [ ] send_files with two files sharing the same basename returns an error before any transfer
• [ ] Old task directories are cleaned up on server restart per retention policy

🤖 Generated with Claude Code

[Anonymous]

Ticket changed by: kumaakh

• status: open --> closed