feat: Vendor beads-merge 3-way merge algorithm (bd-oif6)

- Integrated @neongreen's beads-merge into internal/merge/
- Adapted to use bd's internal/types.Issue instead of custom types
- Added comprehensive tests covering merge scenarios
- Created ATTRIBUTION.md crediting @neongreen
- All tests pass

This solves:
- Multi-workspace deletion sync (bd-hv01)
- Git JSONL merge conflicts
- Field-level intelligent merging

Original: https://github.com/neongreen/mono/tree/main/beads-merge

ATTRIBUTION.md

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+# Attribution and Credits
+
+## beads-merge 3-Way Merge Algorithm
+
+The 3-way merge functionality in `internal/merge/` is based on **beads-merge** by **@neongreen**.
+
+- **Original Repository**: https://github.com/neongreen/mono/tree/main/beads-merge
+- **Author**: @neongreen (https://github.com/neongreen)
+- **Integration Discussion**: https://github.com/neongreen/mono/issues/240
+
+### What We Vendored
+
+The core merge algorithm from beads-merge has been adapted and integrated into bd:
+- Field-level 3-way merge logic
+- Issue identity matching (id + created_at + created_by)
+- Dependency and label merging with deduplication
+- Timestamp handling (max wins)
+- Deletion detection
+- Conflict marker generation
+
+### Changes Made
+
+- Adapted to use bd's `internal/types.Issue` instead of custom types
+- Integrated with bd's JSONL export/import system
+- Added support for bd-specific fields (Design, AcceptanceCriteria, etc.)
+- Exposed as `bd merge` CLI command and library API
+
+### License
+
+The original beads-merge code is used with permission from @neongreen. We are grateful for their contribution to the beads ecosystem.
+
+### Thank You
+
+Special thanks to @neongreen for building beads-merge and graciously allowing us to integrate it into bd. This solves critical multi-workspace sync issues and makes beads much more robust for collaborative workflows.