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

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/anthropics/anthropic-sdk-go v1.16.0
 	github.com/fatih/color v1.18.0
 	github.com/fsnotify/fsnotify v1.9.0
+	github.com/google/go-cmp v0.6.0
 	github.com/ncruces/go-sqlite3 v0.29.1
 	github.com/spf13/cobra v1.10.1
 	github.com/spf13/viper v1.21.0