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/constants/constants.go

@@ -25,8 +25,11 @@ const (
 	// DirBeads is the beads database directory.
 	DirBeads = ".beads"
 
-	// DirGastown is the per-rig runtime state directory.
-	DirGastown = ".gastown"
+	// DirRuntime is the runtime state directory (gitignored).
+	DirRuntime = ".runtime"
+
+	// DirSettings is the rig settings directory (git-tracked).
+	DirSettings = "settings"
 )
 
 // File names for configuration and state.
@@ -132,7 +135,22 @@ func RigCrewPath(rigPath string) string {
 	return rigPath + "/" + DirCrew
 }
 
-// GastownPath returns the path to .gastown/ within a rig.
-func GastownPath(rigPath string) string {
-	return rigPath + "/" + DirGastown
+// MayorConfigPath returns the path to mayor/config.json within a town root.
+func MayorConfigPath(townRoot string) string {
+	return townRoot + "/" + DirMayor + "/" + FileConfigJSON
+}
+
+// TownRuntimePath returns the path to .runtime/ at the town root.
+func TownRuntimePath(townRoot string) string {
+	return townRoot + "/" + DirRuntime
+}
+
+// RigRuntimePath returns the path to .runtime/ within a rig.
+func RigRuntimePath(rigPath string) string {
+	return rigPath + "/" + DirRuntime
+}
+
+// RigSettingsPath returns the path to settings/ within a rig.
+func RigSettingsPath(rigPath string) string {
+	return rigPath + "/" + DirSettings
 }