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/witness/manager.go

@@ -41,7 +41,7 @@ func NewManager(r *rig.Rig) *Manager {
 
 // stateFile returns the path to the witness state file.
 func (m *Manager) stateFile() string {
-	return filepath.Join(m.rig.Path, ".gastown", "witness.json")
+	return filepath.Join(m.rig.Path, ".runtime", "witness.json")
 }
 
 // loadState loads witness state from disk.
@@ -261,7 +261,7 @@ func (m *Manager) checkPolecatHealth(name, path string) PolecatHealthStatus {
 	}
 
 	// Check 2: State file activity
-	stateFile := filepath.Join(path, ".gastown", "state.json")
+	stateFile := filepath.Join(path, ".runtime", "state.json")
 	if info, err := os.Stat(stateFile); err == nil {
 		if time.Since(info.ModTime()) < threshold {
 			return PolecatHealthy