Implement three-tier config architecture (gt-k1lr tasks 1-5)

**Architecture changes:**
- Renamed `.gastown/` → `.runtime/` for runtime state (gitignored)
- Added `settings/` directory for rig behavioral config (git-tracked)
- Added `mayor/config.json` for town-level config (MayorConfig type)
- Separated RigConfig (identity) from RigSettings (behavioral)

**File location changes:**
- Town runtime: `~/.gastown/*` → `~/.runtime/*`
- Rig runtime: `<rig>/.gastown/*` → `<rig>/.runtime/*`
- Rig config: `<rig>/.gastown/config.json` → `<rig>/settings/config.json`
- Namepool state: `namepool.json` → `namepool-state.json`

**New types:**
- MayorConfig: town-level behavioral config
- RigSettings: rig behavioral config (merge_queue, theme, namepool)
- RigConfig now identity-only (name, git_url, beads, created_at)

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

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

internal/cmd/gitinit.go

@@ -79,9 +79,9 @@ const HQGitignore = `# Gas Town HQ .gitignore
 **/crew/
 
 # =============================================================================
-# Rig runtime state directories
+# Runtime state directories (gitignored ephemeral data)
 # =============================================================================
-**/.gastown/
+**/.runtime/
 
 # =============================================================================
 # Rig .beads symlinks (point to ignored mayor/rig/.beads, recreated on setup)