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>

AGENTS.md

@@ -61,6 +61,8 @@ See `integrations/beads-mcp/README.md` for complete documentation.
 
 **RECOMMENDED: Use a single MCP server for all beads projects** - it automatically routes to per-project local daemons.
 
+**For complete multi-repo workflow guide**, see [docs/MULTI_REPO_MIGRATION.md](docs/MULTI_REPO_MIGRATION.md) (OSS contributors, teams, multi-phase development).
+
 **Setup (one-time):**
 ```bash
 # MCP config in ~/.config/amp/settings.json or Claude Desktop config: