Add identity collision prevention, detection, and correction (gt-xp2s)

- internal/lock: New package with PID-based lockfiles for worker identity
- gt prime: Acquire identity lock for crew/polecat roles, fail on collision
- gt agents check: Detect stale locks and identity collisions
- gt agents fix: Clean up stale locks
- gt doctor: New identity-collision check with --fix support

🤖 Generated with [Claude Code](https://claude.ai/code)

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

internal/doctor/identity_check.go

@@ -0,0 +1,136 @@
+package doctor
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/steveyegge/gastown/internal/lock"
+	"github.com/steveyegge/gastown/internal/tmux"
+)
+
+// IdentityCollisionCheck checks for agent identity collisions and stale locks.
+type IdentityCollisionCheck struct{}
+
+// NewIdentityCollisionCheck creates a new identity collision check.
+func NewIdentityCollisionCheck() *IdentityCollisionCheck {
+	return &IdentityCollisionCheck{}
+}
+
+func (c *IdentityCollisionCheck) Name() string {
+	return "identity-collision"
+}
+
+func (c *IdentityCollisionCheck) Description() string {
+	return "Check for agent identity collisions and stale locks"
+}
+
+func (c *IdentityCollisionCheck) CanFix() bool {
+	return true // Can fix stale locks
+}
+
+func (c *IdentityCollisionCheck) Run(ctx *CheckContext) *CheckResult {
+	// Find all locks
+	locks, err := lock.FindAllLocks(ctx.TownRoot)
+	if err != nil {
+		return &CheckResult{
+			Name:    c.Name(),
+			Status:  StatusWarning,
+			Message: fmt.Sprintf("could not scan for locks: %v", err),
+		}
+	}
+
+	if len(locks) == 0 {
+		return &CheckResult{
+			Name:    c.Name(),
+			Status:  StatusOK,
+			Message: "no worker locks found",
+		}
+	}
+
+	// Get active tmux sessions for cross-reference
+	t := tmux.NewTmux()
+	sessions, _ := t.ListSessions() // Ignore errors - might not have tmux
+
+	sessionSet := make(map[string]bool)
+	for _, s := range sessions {
+		sessionSet[s] = true
+	}
+
+	var staleLocks []string
+	var orphanedLocks []string
+	var healthyLocks int
+
+	for workerDir, info := range locks {
+		if info.IsStale() {
+			staleLocks = append(staleLocks,
+				fmt.Sprintf("%s (dead PID %d)", workerDir, info.PID))
+			continue
+		}
+
+		// Check if session exists
+		if info.SessionID != "" && !sessionSet[info.SessionID] {
+			// Lock has session ID but session doesn't exist
+			// This could be a collision or orphan
+			orphanedLocks = append(orphanedLocks,
+				fmt.Sprintf("%s (PID %d, missing session %s)", workerDir, info.PID, info.SessionID))
+			continue
+		}
+
+		healthyLocks++
+	}
+
+	// Build result
+	if len(staleLocks) == 0 && len(orphanedLocks) == 0 {
+		return &CheckResult{
+			Name:    c.Name(),
+			Status:  StatusOK,
+			Message: fmt.Sprintf("%d worker lock(s), all healthy", healthyLocks),
+		}
+	}
+
+	result := &CheckResult{
+		Name: c.Name(),
+	}
+
+	if len(staleLocks) > 0 {
+		result.Status = StatusWarning
+		result.Message = fmt.Sprintf("%d stale lock(s) found", len(staleLocks))
+		result.Details = append(result.Details, "Stale locks (dead PIDs):")
+		for _, s := range staleLocks {
+			result.Details = append(result.Details, "  "+s)
+		}
+		result.FixHint = "Run 'gt doctor --fix' or 'gt agents fix' to clean up"
+	}
+
+	if len(orphanedLocks) > 0 {
+		if result.Status != StatusWarning {
+			result.Status = StatusWarning
+		}
+		if result.Message != "" {
+			result.Message += ", "
+		}
+		result.Message += fmt.Sprintf("%d orphaned lock(s)", len(orphanedLocks))
+		result.Details = append(result.Details, "Orphaned locks (missing sessions):")
+		for _, s := range orphanedLocks {
+			result.Details = append(result.Details, "  "+s)
+		}
+		if !strings.Contains(result.FixHint, "doctor") {
+			result.FixHint = "Run 'gt doctor --fix' to clean up stale locks"
+		}
+	}
+
+	return result
+}
+
+func (c *IdentityCollisionCheck) Fix(ctx *CheckContext) error {
+	cleaned, err := lock.CleanStaleLocks(ctx.TownRoot)
+	if err != nil {
+		return fmt.Errorf("cleaning stale locks: %w", err)
+	}
+
+	if cleaned > 0 {
+		fmt.Printf("  Cleaned %d stale lock(s)\n", cleaned)
+	}
+
+	return nil
+}