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/polecat/namepool_test.go

@@ -308,7 +308,7 @@ func TestNamePool_StateFilePath(t *testing.T) {
 	}
 
 	// Verify file was created in expected location
-	expectedPath := filepath.Join(tmpDir, ".gastown", "namepool.json")
+	expectedPath := filepath.Join(tmpDir, ".runtime", "namepool-state.json")
 	if _, err := os.Stat(expectedPath); err != nil {
 		t.Errorf("state file not found at expected path: %v", err)
 	}