docs: add harness design documentation (gt-cr9)

Create comprehensive harness documentation covering:
- What a harness is (installation directory vs town concept)
- Harness structure and creation with gt install
- Beads redirect patterns for complex setups
- Multi-system sharing (PGT/GGT coexistence)
- Configuration files (town.json, rigs.json, state.json)
- Harness templates for organizations
- Migration between harnesses

Update architecture.md to:
- Rename "Town" section to "Harness (Town)" with clearer explanation
- Add cross-references to new harness.md
- Update directory structure section with "Harness Level" heading
- Expand CLI commands section with more gt install options

Update install.go help text to reference harness.md.

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

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

internal/cmd/install.go

@@ -30,14 +30,18 @@ var installCmd = &cobra.Command{
 	Short: "Create a new Gas Town harness (workspace)",
 	Long: `Create a new Gas Town harness at the specified path.
 
-A harness is the top-level directory where Gas Town is installed. It contains:
+A harness is the top-level directory where Gas Town is installed - the root of
+your workspace where all rigs and agents live. It contains:
   - CLAUDE.md            Mayor role context (Mayor runs from harness root)
-  - mayor/               Mayor config and state
-  - rigs/                Managed rig clones (created by 'gt rig add')
-  - .beads/              Town-level beads DB (mayor mail, coordination)
+  - mayor/               Mayor config, state, and rig registry
+  - rigs/                Managed rig containers (created by 'gt rig add')
+  - .beads/              Town-level beads DB (gm-* prefix for mayor mail)
 
 If path is omitted, uses the current directory.
 
+See docs/harness.md for advanced harness configurations including beads
+redirects, multi-system setups, and harness templates.
+
 Examples:
   gt install ~/gt                         # Create harness at ~/gt
   gt install . --name my-workspace        # Initialize current dir