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

.gitignore

@@ -114,7 +114,7 @@ bd-trace-*.out
 # AI planning documents (ephemeral)
 history/
 
-# Gas Town agent state
+# Orchestrator agent state
 state.json
 .beads/redirect