feat: add graceful pause (drain mode) for running agents

File-based signal (.pause_drain) lets the orchestrator finish current
work before pausing instead of hard-freezing the process tree.  New
status states pausing/paused_graceful flow through WebSocket to the UI
where a Pause button, draining indicator, and Resume button are shown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/package-lock.json

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