Replace ProcessExists() checks in witness and refinery managers with
tmux session detection. Agent liveness should be derived from tmux
session state, not PID probing (per ZFC tracking principles).
- Remove util.ProcessExists() from witness/manager.go and refinery/manager.go
- Delete internal/util/process.go and process_test.go (now unused)
- Foreground mode and Stop() now rely solely on tmux HasSession/KillSession
Closes: hq-yxkdr (recentDeaths already removed)
Closes: hq-1sd4o (ProcessExists removed)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>