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/architecture.md

@@ -11,7 +11,7 @@ graph TB
     subgraph "Gas Town"
         Overseer["👤 Overseer<br/>(Human Operator)"]
 
-        subgraph Town["Town (~/ai/)"]
+        subgraph Town["Town (~/gt/)"]
             Mayor["🎩 Mayor<br/>(Global Coordinator)"]
 
             subgraph Rig1["Rig: wyvern"]
@@ -45,6 +45,10 @@ graph TB
 
 ## Core Concepts
 
+### Harness
+
+A **Harness** is the installation directory for Gas Town - the top-level workspace containing all components. The canonical GGT harness is at `~/gt/`. See [harness.md](harness.md) for setup details and migration from legacy configurations.
+
 ### Town
 
 A **Town** is a complete Gas Town installation - the workspace where everything lives. A town contains:
@@ -1281,7 +1285,7 @@ type Worker interface {
 ### Configuration
 
 ```yaml
-# ~/ai/config/outposts.yaml
+# ~/gt/config/outposts.yaml
 outposts:
   - name: local
     type: local