feat(doctor): add hook-attachment-valid check (gt-h6eq.2)

Add doctor check to verify that attached molecules exist and are not closed.
Detects when a hook's attached_molecule field points to a non-existent or
closed issue, which can leave agents with stale work assignments.

- Check: hook-attachment-valid
- Error if: Hook's attached_molecule field points to non-existent or closed issue
- Fix: Automatically detaches invalid molecules when run with --fix

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/doctor/hook_check_test.go

@@ -0,0 +1,123 @@
+package doctor
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestNewHookAttachmentValidCheck(t *testing.T) {
+	check := NewHookAttachmentValidCheck()
+
+	if check.Name() != "hook-attachment-valid" {
+		t.Errorf("expected name 'hook-attachment-valid', got %q", check.Name())
+	}
+
+	if check.Description() != "Verify attached molecules exist and are not closed" {
+		t.Errorf("unexpected description: %q", check.Description())
+	}
+
+	if !check.CanFix() {
+		t.Error("expected CanFix to return true")
+	}
+}
+
+func TestHookAttachmentValidCheck_NoBeadsDir(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	check := NewHookAttachmentValidCheck()
+	ctx := &CheckContext{TownRoot: tmpDir}
+
+	result := check.Run(ctx)
+
+	// No beads dir means nothing to check, should be OK
+	if result.Status != StatusOK {
+		t.Errorf("expected StatusOK when no beads dir, got %v", result.Status)
+	}
+}
+
+func TestHookAttachmentValidCheck_EmptyBeadsDir(t *testing.T) {
+	tmpDir := t.TempDir()
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	check := NewHookAttachmentValidCheck()
+	ctx := &CheckContext{TownRoot: tmpDir}
+
+	result := check.Run(ctx)
+
+	// Empty beads dir means no pinned beads, should be OK
+	// Note: This may error if bd CLI is not available, but should still handle gracefully
+	if result.Status != StatusOK && result.Status != StatusError {
+		t.Errorf("expected StatusOK or graceful error, got %v", result.Status)
+	}
+}
+
+func TestHookAttachmentValidCheck_FormatInvalid(t *testing.T) {
+	check := NewHookAttachmentValidCheck()
+
+	tests := []struct {
+		inv      invalidAttachment
+		expected string
+	}{
+		{
+			inv: invalidAttachment{
+				pinnedBeadID: "hq-123",
+				moleculeID:   "gt-456",
+				reason:       "not_found",
+			},
+			expected: "hq-123: attached molecule gt-456 not found",
+		},
+		{
+			inv: invalidAttachment{
+				pinnedBeadID: "hq-123",
+				moleculeID:   "gt-789",
+				reason:       "closed",
+			},
+			expected: "hq-123: attached molecule gt-789 is closed",
+		},
+	}
+
+	for _, tt := range tests {
+		result := check.formatInvalid(tt.inv)
+		if result != tt.expected {
+			t.Errorf("formatInvalid() = %q, want %q", result, tt.expected)
+		}
+	}
+}
+
+func TestHookAttachmentValidCheck_FindRigBeadsDirs(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	// Create town-level .beads (should be excluded)
+	townBeads := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(townBeads, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create rig-level .beads
+	rigBeads := filepath.Join(tmpDir, "myrig", ".beads")
+	if err := os.MkdirAll(rigBeads, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	check := NewHookAttachmentValidCheck()
+	dirs := check.findRigBeadsDirs(tmpDir)
+
+	// Should find the rig-level beads but not town-level
+	found := false
+	for _, dir := range dirs {
+		if dir == townBeads {
+			t.Error("findRigBeadsDirs should not include town-level .beads")
+		}
+		if dir == rigBeads {
+			found = true
+		}
+	}
+
+	if !found && len(dirs) > 0 {
+		t.Logf("Found dirs: %v", dirs)
+	}
+}