refactor: remove Gas Town references from codebase

Replace Gas Town-specific terminology with generic orchestrator concepts:
- "Gas Town" → "orchestrator" or "multi-clone"
- Hardcoded ~/gt/ paths → GT_ROOT environment variable
- signalGasTownActivity → signalOrchestratorActivity
- GUPP → hook-based work assignment

Updated 21 files across CHANGELOG, cmd/bd/, internal/, docs/, scripts.

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/formula.go

@@ -32,7 +32,7 @@ The Rig → Cook → Run lifecycle:
 Search paths (in order):
   1. .beads/formulas/ (project)
   2. ~/.beads/formulas/ (user)
-  3. ~/gt/.beads/formulas/ (town)
+  3. $GT_ROOT/.beads/formulas/ (orchestrator, if GT_ROOT set)
 
 Commands:
   list   List available formulas from all search paths
@@ -48,7 +48,7 @@ var formulaListCmd = &cobra.Command{
 Search paths (in order of priority):
   1. .beads/formulas/ (project - highest priority)
   2. ~/.beads/formulas/ (user)
-  3. ~/gt/.beads/formulas/ (town)
+  3. $GT_ROOT/.beads/formulas/ (orchestrator, if GT_ROOT set)
 
 Formulas in earlier paths shadow those with the same name in later paths.
 
@@ -359,8 +359,11 @@ func getFormulaSearchPaths() []string {
 	// User-level formulas
 	if home, err := os.UserHomeDir(); err == nil {
 		paths = append(paths, filepath.Join(home, ".beads", "formulas"))
-		// Gas Town formulas
-		paths = append(paths, filepath.Join(home, "gt", ".beads", "formulas"))
+	}
+
+	// Orchestrator formulas (via GT_ROOT)
+	if gtRoot := os.Getenv("GT_ROOT"); gtRoot != "" {
+		paths = append(paths, filepath.Join(gtRoot, ".beads", "formulas"))
 	}
 
 	return paths