fix(zfc): remove PID-based agent liveness detection

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>

internal/witness/manager.go

@@ -16,7 +16,6 @@ import (
 	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/tmux"
-	"github.com/steveyegge/gastown/internal/util"
 	"github.com/steveyegge/gastown/internal/workspace"
 )
 
@@ -114,14 +113,15 @@ func (m *Manager) Start(foreground bool, agentOverride string, envOverrides []st
 
 	if foreground {
 		// Foreground mode is deprecated - patrol logic moved to mol-witness-patrol
-		if w.State == StateRunning && w.PID > 0 && util.ProcessExists(w.PID) {
+		// Just check tmux session (no PID inference per ZFC)
+		if running, _ := t.HasSession(sessionID); running && t.IsClaudeRunning(sessionID) {
 			return ErrAlreadyRunning
 		}
 
 		now := time.Now()
 		w.State = StateRunning
 		w.StartedAt = &now
-		w.PID = os.Getpid()
+		w.PID = 0 // No longer track PID (ZFC)
 		w.MonitoredPolecats = m.rig.Polecats
 
 		return m.saveState(w)
@@ -141,10 +141,7 @@ func (m *Manager) Start(foreground bool, agentOverride string, envOverrides []st
 		}
 	}
 
-	// Also check via PID for backwards compatibility
-	if w.State == StateRunning && w.PID > 0 && util.ProcessExists(w.PID) {
-		return ErrAlreadyRunning
-	}
+	// Note: No PID check per ZFC - tmux session is the source of truth
 
 	// Working directory
 	witnessDir := m.witnessDir()
@@ -320,13 +317,7 @@ func (m *Manager) Stop() error {
 		_ = t.KillSession(sessionID)
 	}
 
-	// If we have a PID and it's a different process, try to stop it gracefully
-	if w.PID > 0 && w.PID != os.Getpid() && util.ProcessExists(w.PID) {
-		// Send SIGTERM (best-effort graceful stop)
-		if proc, err := os.FindProcess(w.PID); err == nil {
-			_ = proc.Signal(os.Interrupt)
-		}
-	}
+	// Note: No PID-based stop per ZFC - tmux session kill is sufficient
 
 	w.State = StateStopped
 	w.PID = 0