fix(spawn): use rig-level beads instead of mayor/rig path

The spawn command and polecat manager were incorrectly looking for beads
in <rig>/mayor/rig/.beads instead of <rig>/.beads. This caused:
- `gt spawn --issue ga-xxp` to fail with "no issue found" even when
  `bd show ga-xxp` worked from the rig directory
- Polecats couldn't find rig-level issues (with ga- prefix) because
  mayor/rig has a different beads database (with gt- prefix)

This fix:
- spawn.go: Use r.Path instead of filepath.Join(r.Path, "mayor", "rig")
- polecat/manager.go: Use r.Path for beads.New() initialization
- session/manager.go: Use r.Path/.beads for BEADS_DIR environment

Closes beads-2nh

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

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

internal/session/manager.go

@@ -128,9 +128,9 @@ func (m *Manager) Start(polecat string, opts StartOptions) error {
 	_ = m.tmux.SetEnvironment(sessionID, "GT_POLECAT", polecat)
 
 	// CRITICAL: Set beads environment for worktree polecats
-	// Polecats share the rig's beads directory (in mayor/rig/.beads)
+	// Polecats share the rig's beads directory (at rig root, not mayor/rig)
 	// BEADS_NO_DAEMON=1 prevents daemon from committing to wrong branch
-	beadsDir := filepath.Join(m.rig.Path, "mayor", "rig", ".beads")
+	beadsDir := filepath.Join(m.rig.Path, ".beads")
 	_ = m.tmux.SetEnvironment(sessionID, "BEADS_DIR", beadsDir)
 	_ = m.tmux.SetEnvironment(sessionID, "BEADS_NO_DAEMON", "1")
 	_ = m.tmux.SetEnvironment(sessionID, "BEADS_AGENT_NAME", fmt.Sprintf("%s/%s", m.rig.Name, polecat))