/workflows command (list, show, kill, pause)

Add /workflows command backed by the WorkflowTask registry:

- list: running + recent tasks
- show <id>: meta, snapshot, logs
- kill <id>: abort via task.abortController
- pause <id>: mark paused, persist task.json, stop further agents (no step replay yet)

Match Claude Code task UI affordances (skip/retry per agent come later).

Parent: https://github.com/Michaelliv/pi-dynamic-workflows/issues/1