Add multi-repo patterns documentation for AI agents (bd-4b6u)

- Create docs/MULTI_REPO_AGENTS.md with comprehensive multi-repo patterns
- Document config options, routing behavior, troubleshooting, best practices
- Update AGENTS.md to reference new doc
- Update README.md documentation index

Amp-Thread-ID: https://ampcode.com/threads/T-645ccbf4-a5b6-44a1-bbd9-913447e17b49
Co-authored-by: Amp <amp@ampcode.com>

README.md

@@ -675,6 +675,7 @@ For advanced usage, see:
 - **[INSTALLING.md](INSTALLING.md)** - Complete installation guide for all platforms
 - **[QUICKSTART.md](QUICKSTART.md)** - Interactive tutorial (`bd quickstart`)
 - **[docs/MULTI_REPO_MIGRATION.md](docs/MULTI_REPO_MIGRATION.md)** - Multi-repo workflow guide (OSS, teams, multi-phase)
+- **[docs/MULTI_REPO_AGENTS.md](docs/MULTI_REPO_AGENTS.md)** - Multi-repo patterns for AI agents
 - **[FAQ.md](FAQ.md)** - Frequently asked questions
 - **[TROUBLESHOOTING.md](TROUBLESHOOTING.md)** - Common issues and solutions
 - **[ADVANCED.md](ADVANCED.md)** - Advanced features and use cases