Add attach/detach functionality to falcon launch

[cweniger]

Summary

Consolidate falcon monitor functionality into falcon launch by adding attach/detach capability. This would allow users to:

• Detach from a running falcon launch session while keeping training running
• Re-attach later to see current progress
• Eliminate the need for a separate falcon monitor command

Background

The interactive TUI mode was added to falcon launch (see recent commits on feat/improved_output branch). Currently, falcon monitor is a separate Textual-based TUI that connects to running falcon processes via Ray actors (MonitorBridge).

Proposed Design

Detach

• User presses d key during interactive mode
• Training continues in background (Ray actors keep running)
• Terminal is released back to user
• MonitorBridge actor remains accessible for re-attachment

Attach

• falcon launch --attach or falcon attach
• Discovers running falcon process via ray.get_actor("falcon:monitor_bridge")
• Reconnects interactive TUI to show current state
• Resumes keyboard control (j/k for node switching, Ctrl+C for stop)

Implementation Notes

• The MonitorBridge actor already provides all necessary status methods
• Interactive display already reads node logs from disk (works for attach)
• May need to persist some state (e.g., which nodes exist) for clean attach
• Consider: should attach support filtering to specific run-dir?

Related

• falcon monitor can be deprecated once this is implemented
• Currently hidden from help via recent commit