fix(beads): multi-repo routing for custom types and role slots

Fixes two bugs in multi-repo routing scenarios:

1. "invalid issue type: agent" error when creating agent beads
   - Added EnsureCustomTypes() with two-level caching (in-memory + sentinel file)
   - CreateAgentBead() now resolves routing target and ensures custom types

2. "could not set role slot: issue not found" warning when setting slots
   - Added runSlotSet() and runSlotClear() helpers that run bd from correct directory
   - Slot operations now use the resolved target directory

New files:
- internal/beads/beads_types.go - routing resolution and custom types logic
- internal/beads/beads_types_test.go - unit tests

Based on PR #811 by Perttulands, rebased onto current main.

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

internal/beads/beads.go

@@ -114,6 +114,11 @@ type Beads struct {
 	workDir  string
 	beadsDir string // Optional BEADS_DIR override for cross-database access
 	isolated bool   // If true, suppress inherited beads env vars (for test isolation)
+
+	// Lazy-cached town root for routing resolution.
+	// Populated on first call to getTownRoot() to avoid filesystem walk on every operation.
+	townRoot     string
+	searchedRoot bool
 }
 
 // New creates a new Beads wrapper for the given directory.
@@ -144,6 +149,26 @@ func (b *Beads) getActor() string {
 	return os.Getenv("BD_ACTOR")
 }
 
+// getTownRoot returns the Gas Town root directory, using lazy caching.
+// The town root is found by walking up from workDir looking for mayor/town.json.
+// Returns empty string if not in a Gas Town project.
+func (b *Beads) getTownRoot() string {
+	if !b.searchedRoot {
+		b.townRoot = FindTownRoot(b.workDir)
+		b.searchedRoot = true
+	}
+	return b.townRoot
+}
+
+// getResolvedBeadsDir returns the beads directory this wrapper is operating on.
+// This follows any redirects and returns the actual beads directory path.
+func (b *Beads) getResolvedBeadsDir() string {
+	if b.beadsDir != "" {
+		return b.beadsDir
+	}
+	return ResolveBeadsDir(b.workDir)
+}
+
 // Init initializes a new beads database in the working directory.
 // This uses the same environment isolation as other commands.
 func (b *Beads) Init(prefix string) error {