feat: add .beads-ephemeral/ creation to gt rig add

During rig initialization, now creates a .beads-ephemeral/ directory:
- Initialized as a git repo (for local versioning)
- Contains config.yaml with ephemeral: true
- Automatically added to rig .gitignore

This provides a local-only beads database for runtime tracking of
wisps and molecules, separate from the synced .beads/ database.

Closes gt-3x0z.1

Generated with Claude Code

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

internal/cmd/rig.go

@@ -43,6 +43,7 @@ var rigAddCmd = &cobra.Command{
 This creates a rig container with:
   - config.json           Rig configuration
   - .beads/               Rig-level issue tracking (initialized)
+  - .beads-ephemeral/     Local runtime tracking (gitignored)
   - refinery/rig/         Canonical main clone
   - mayor/rig/            Mayor's working clone
   - crew/main/            Default human workspace
@@ -192,6 +193,7 @@ func runRigAdd(cmd *cobra.Command, args []string) error {
 	fmt.Printf("  %s/\n", name)
 	fmt.Printf("  ├── config.json\n")
 	fmt.Printf("  ├── .beads/           (prefix: %s)\n", newRig.Config.Prefix)
+	fmt.Printf("  ├── .beads-ephemeral/ (local runtime tracking)\n")
 	fmt.Printf("  ├── refinery/rig/     (canonical main)\n")
 	fmt.Printf("  ├── mayor/rig/        (mayor's clone)\n")
 	fmt.Printf("  ├── crew/%s/        (your workspace)\n", rigAddCrew)