feat(deps): detect/prevent child→parent dependency anti-pattern (bd-nim5)

This prevents a common mistake where users add dependencies from child
issues to their parent epics. This creates a deadlock:
- Child can't start (blocked by open parent)
- Parent can't close (children not done)

Changes:
- dep.go: Reject child→parent deps at creation time with clear error
- server_labels_deps_comments.go: Same check for daemon RPC
- doctor/validation.go: New check detects existing bad deps
- doctor/fix/validation.go: Auto-fix removes bad deps
- doctor.go: Wire up check and fix handler

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

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

cmd/bd/doctor/fix/validation_test.go

@@ -1,7 +1,12 @@
 package fix
 
 import (
+	"os"
+	"path/filepath"
 	"testing"
+
+	_ "github.com/ncruces/go-sqlite3/driver"
+	_ "github.com/ncruces/go-sqlite3/embed"
 )
 
 // TestFixFunctions_RequireBeadsDir verifies all fix functions properly validate
@@ -22,6 +27,7 @@ func TestFixFunctions_RequireBeadsDir(t *testing.T) {
 		{"SyncBranchHealth", func(dir string) error { return SyncBranchHealth(dir, "beads-sync") }},
 		{"UntrackedJSONL", UntrackedJSONL},
 		{"MigrateTombstones", MigrateTombstones},
+		{"ChildParentDependencies", ChildParentDependencies},
 	}
 
 	for _, tc := range funcs {
@@ -35,3 +41,100 @@ func TestFixFunctions_RequireBeadsDir(t *testing.T) {
 		})
 	}
 }
+
+func TestChildParentDependencies_NoBadDeps(t *testing.T) {
+	// Set up test database with no child→parent deps
+	dir := t.TempDir()
+	beadsDir := filepath.Join(dir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	dbPath := filepath.Join(beadsDir, "beads.db")
+	db, err := openDB(dbPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create minimal schema
+	_, err = db.Exec(`
+		CREATE TABLE issues (id TEXT PRIMARY KEY);
+		CREATE TABLE dependencies (issue_id TEXT, depends_on_id TEXT, type TEXT);
+		CREATE TABLE dirty_issues (issue_id TEXT PRIMARY KEY);
+		INSERT INTO issues (id) VALUES ('bd-abc'), ('bd-abc.1'), ('bd-xyz');
+		INSERT INTO dependencies (issue_id, depends_on_id, type) VALUES ('bd-abc.1', 'bd-xyz', 'blocks');
+	`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	db.Close()
+
+	// Run fix - should find no bad deps
+	err = ChildParentDependencies(dir)
+	if err != nil {
+		t.Errorf("ChildParentDependencies failed: %v", err)
+	}
+
+	// Verify the good dependency still exists
+	db, _ = openDB(dbPath)
+	defer db.Close()
+	var count int
+	db.QueryRow("SELECT COUNT(*) FROM dependencies").Scan(&count)
+	if count != 1 {
+		t.Errorf("Expected 1 dependency, got %d", count)
+	}
+}
+
+func TestChildParentDependencies_FixesBadDeps(t *testing.T) {
+	// Set up test database with child→parent deps
+	dir := t.TempDir()
+	beadsDir := filepath.Join(dir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	dbPath := filepath.Join(beadsDir, "beads.db")
+	db, err := openDB(dbPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create minimal schema with child→parent dependency
+	_, err = db.Exec(`
+		CREATE TABLE issues (id TEXT PRIMARY KEY);
+		CREATE TABLE dependencies (issue_id TEXT, depends_on_id TEXT, type TEXT);
+		CREATE TABLE dirty_issues (issue_id TEXT PRIMARY KEY);
+		INSERT INTO issues (id) VALUES ('bd-abc'), ('bd-abc.1'), ('bd-abc.1.2');
+		INSERT INTO dependencies (issue_id, depends_on_id, type) VALUES
+			('bd-abc.1', 'bd-abc', 'blocks'),
+			('bd-abc.1.2', 'bd-abc', 'blocks'),
+			('bd-abc.1.2', 'bd-abc.1', 'blocks');
+	`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	db.Close()
+
+	// Run fix
+	err = ChildParentDependencies(dir)
+	if err != nil {
+		t.Errorf("ChildParentDependencies failed: %v", err)
+	}
+
+	// Verify all bad dependencies were removed
+	db, _ = openDB(dbPath)
+	defer db.Close()
+	var count int
+	db.QueryRow("SELECT COUNT(*) FROM dependencies").Scan(&count)
+	if count != 0 {
+		t.Errorf("Expected 0 dependencies after fix, got %d", count)
+	}
+
+	// Verify dirty_issues was updated for affected issues
+	// Note: 2 unique issue_ids (bd-abc.1 appears once, bd-abc.1.2 appears twice but INSERT OR IGNORE dedupes)
+	var dirtyCount int
+	db.QueryRow("SELECT COUNT(*) FROM dirty_issues").Scan(&dirtyCount)
+	if dirtyCount != 2 {
+		t.Errorf("Expected 2 dirty issues (unique issue_ids from removed deps), got %d", dirtyCount)
+	}
+}