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/mol_squash.go

@@ -34,7 +34,7 @@ The squash operation:
 
 AGENT INTEGRATION:
 Use --summary to provide an AI-generated summary. This keeps bd as a pure
-tool - the calling agent (Gas Town polecat, Claude Code, etc.) is responsible
+tool - the calling agent (orchestrator worker, Claude Code, etc.) is responsible
 for generating intelligent summaries. Without --summary, a basic concatenation
 of child issue content is used.