Add multi-repo migration guide and bd migrate-issues command

- Created comprehensive migration guide at docs/MULTI_REPO_MIGRATION.md
  - Covers OSS contributor, team, multi-phase, and persona workflows
  - Step-by-step setup instructions with examples
  - Configuration reference and troubleshooting
- Implemented bd migrate-issues command
  - Move issues between repos with filtering (status/priority/labels/type)
  - Dependency preservation with upstream/downstream/closure options
  - Dry-run mode and strict validation
  - Interactive confirmation with --yes override
- Updated README.md and AGENTS.md with migration guide links

Completes: bd-c3ei, bd-mlcz
Part of epic: bd-8rd (Migration and onboarding for multi-repo)

Amp-Thread-ID: https://ampcode.com/threads/T-c5a7a780-b05e-4cc3-a7c1-5de107821b7e
Co-authored-by: Amp <amp@ampcode.com>

README.md

@@ -660,6 +660,7 @@ For advanced usage, see:
 - **[README.md](README.md)** - You are here! Core features and quick start
 - **[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)
 - **[FAQ.md](FAQ.md)** - Frequently asked questions
 - **[TROUBLESHOOTING.md](TROUBLESHOOTING.md)** - Common issues and solutions
 - **[ADVANCED.md](ADVANCED.md)** - Advanced features and use cases