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

docs/CLI_REFERENCE.md

@@ -574,7 +574,7 @@ bd sync
 - `tombstone` - Deleted issue (suppresses resurrections)
 - `pinned` - Stays open indefinitely (used for hooks, anchors)
 
-**Note:** The `pinned` status is used by Gas Town for hook management and persistent work items that should never be auto-closed or cleaned up.
+**Note:** The `pinned` status is used by orchestrators for hook management and persistent work items that should never be auto-closed or cleaned up.
 
 ## Priorities