Add docs/harness.md covering:
- What a harness (installation directory) is
- Directory structure with mayor/ config files
- Town-level vs rig-level mayor presence
- Beads architecture and resolution via BEADS_DIR
- Relationship between harness and rigs
- Example configurations and setup workflow
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
