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.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

docs/MOLECULES.md

@@ -113,7 +113,7 @@ bd mol bond A B --type conditional # B runs only if A fails
 - Agent can continue from A into B seamlessly
 - The compound work graph can span days
 
-This is how Gas Town runs autonomous workflows - agents follow the dependency graph, handing off between sessions, until all work closes.
+This is how orchestrators run autonomous workflows - agents follow the dependency graph, handing off between sessions, until all work closes.
 
 ## Phase Metaphor (Templates)