fix(sling): Set BEADS_DIR for accessing hq-* beads from polecat worktree

When running bd update commands for hq-* beads from a polecat worktree,
the redirect mechanism only exposes gt-* beads. This fix sets BEADS_DIR
to the town-level .beads directory so hq-* beads are accessible.

Also adds NewWithBeadsDir() constructor to beads package for explicit
cross-database access when needed.

internal/beads/beads.go

@@ -209,7 +209,8 @@ type SyncStatus struct {
 
 // Beads wraps bd CLI operations for a working directory.
 type Beads struct {
-	workDir string
+	workDir  string
+	beadsDir string // Optional BEADS_DIR override for cross-database access
 }
 
 // New creates a new Beads wrapper for the given directory.
@@ -217,6 +218,12 @@ func New(workDir string) *Beads {
 	return &Beads{workDir: workDir}
 }
 
+// NewWithBeadsDir creates a Beads wrapper with an explicit BEADS_DIR.
+// This is needed when running from a polecat worktree but accessing town-level beads.
+func NewWithBeadsDir(workDir, beadsDir string) *Beads {
+	return &Beads{workDir: workDir, beadsDir: beadsDir}
+}
+
 // run executes a bd command and returns stdout.
 func (b *Beads) run(args ...string) ([]byte, error) {
 	// Use --no-daemon for faster read operations (avoids daemon IPC overhead)
@@ -225,6 +232,11 @@ func (b *Beads) run(args ...string) ([]byte, error) {
 	cmd := exec.Command("bd", fullArgs...)
 	cmd.Dir = b.workDir
 
+	// Set BEADS_DIR if specified (enables cross-database access)
+	if b.beadsDir != "" {
+		cmd.Env = append(os.Environ(), "BEADS_DIR="+b.beadsDir)
+	}
+
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr