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/migrate_sync.go

@@ -20,8 +20,8 @@ var migrateSyncCmd = &cobra.Command{
 
 This command configures the repository to commit .beads changes to a separate
 branch (e.g., "beads-sync") instead of the current working branch. This is
-essential for multi-clone setups like Gas Town where multiple clones work
-independently but need to sync beads data.
+essential for multi-clone setups where multiple clones work independently
+but need to sync beads data.
 
 The command will:
   1. Validate the current state (not already configured, not on sync branch)