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/activity.go

@@ -456,7 +456,7 @@ func discoverRigDaemons() []rigDaemon {
 	// Find town beads directory (uses findTownBeadsDir from create.go)
 	townBeadsDir, err := findTownBeadsDir()
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "Error: not in a Gas Town (%v)\n", err)
+		fmt.Fprintf(os.Stderr, "Error: not in an orchestrator environment (%v)\n", err)
 		os.Exit(1)
 	}