docs: document GGT harness separation from PGT (gt-dq3)

Created docs/harness.md documenting the GGT harness at ~/gt/ and its
separation from the legacy PGT harness at ~/ai/. Updated architecture.md
and federation-design.md to use ~/gt/ consistently.

Key changes:
- Created harness.md with full harness structure documentation
- Added Harness section to Core Concepts in architecture.md
- Updated mermaid diagrams from ~/ai/ to ~/gt/
- Documented cleanup steps for legacy ~/ai/ artifacts

🤝 Handoff: The ~/ai/ directory has GGT leftovers that can be cleaned:
- rm -rf ~/ai/gastown/mayor/ (old GGT clone)
- rm ~/ai/.beads/redirect (old beads redirect)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/federation-design.md

@@ -12,13 +12,13 @@ Gas Town needs to scale beyond a single machine:
 
 ### Model A: "Town Clone" (VMs)
 
-Clone the entire `~/ai` workspace to a remote VM. It runs like a regular Gas Town:
+Clone the entire `~/gt` workspace to a remote VM. It runs like a regular Gas Town:
 
 ```
 ┌─────────────────────────────────────────┐
 │  GCE VM (or any Linux box)              │
 │                                         │
-│  ~/ai/                # Full town clone │
+│  ~/gt/                # Full town clone │
 │  ├── config/          # Town config     │
 │  ├── mayor/           # Mayor (or none) │
 │  ├── gastown/         # Rig with agents │
@@ -356,7 +356,7 @@ Workers need Claude API access:
 ## Configuration
 
 ```yaml
-# ~/ai/config/outposts.yaml
+# ~/gt/config/outposts.yaml
 outposts:
   # Always present - the local machine
   - name: local