fix(sling): use bd native prefix routing instead of BEADS_DIR override

verifyBeadExists was setting BEADS_DIR to town root, which overrides
bd's native prefix-based routing via routes.jsonl. This broke resolution
of rig-level beads (e.g., gt-* beads routed via gt- -> gastown/mayor/rig).

Fix:
- Remove BEADS_DIR override in verifyBeadExists
- Set cmd.Dir to town root so bd can find routes.jsonl
- Apply same fix to getBeadInfo for consistency

Now gt sling gt-xxx correctly finds beads using the same routing as
bd show gt-xxx.

(gt-l5qwb)

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

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

internal/cmd/sling.go

@@ -680,11 +680,13 @@ func sessionToAgentID(sessionName string) string {
 }
 
 // verifyBeadExists checks that the bead exists using bd show.
+// Uses bd's native prefix-based routing via routes.jsonl - do NOT set BEADS_DIR
+// as that overrides routing and breaks resolution of rig-level beads.
 func verifyBeadExists(beadID string) error {
 	cmd := exec.Command("bd", "--no-daemon", "show", beadID, "--json")
-	// Set BEADS_DIR to town root so hq-* beads are accessible
+	// Run from town root so bd can find routes.jsonl for prefix-based routing.
+	// Do NOT set BEADS_DIR - that overrides routing and breaks rig bead resolution.
 	if townRoot, err := workspace.FindFromCwd(); err == nil {
-		cmd.Env = append(os.Environ(), "BEADS_DIR="+filepath.Join(townRoot, ".beads"))
 		cmd.Dir = townRoot
 	}
 	if err := cmd.Run(); err != nil {
@@ -701,8 +703,13 @@ type beadInfo struct {
 }
 
 // getBeadInfo returns status and assignee for a bead.
+// Uses bd's native prefix-based routing via routes.jsonl.
 func getBeadInfo(beadID string) (*beadInfo, error) {
 	cmd := exec.Command("bd", "--no-daemon", "show", beadID, "--json")
+	// Run from town root so bd can find routes.jsonl for prefix-based routing.
+	if townRoot, err := workspace.FindFromCwd(); err == nil {
+		cmd.Dir = townRoot
+	}
 	out, err := cmd.Output()
 	if err != nil {
 		return nil, fmt.Errorf("bead '%s' not found", beadID)