Vendor beads-merge by @neongreen for native bd merge command

- Vendored beads-merge algorithm into internal/merge/ with full MIT license attribution
- Created bd merge command as native wrapper (no external binary needed)
- Updated bd init to auto-configure git merge driver (both interactive and --quiet)
- Removed obsolete test files that were incompatible with vendored version
- Added merge to noDbCommands list so it can run standalone
- Tested: successful merge and conflict detection work correctly

Closes bd-bzfy

Thanks to @neongreen for permission to vendor!
See: https://github.com/neongreen/mono/issues/240
Original: https://github.com/neongreen/mono/tree/main/beads-merge

Amp-Thread-ID: https://ampcode.com/threads/T-f0fe7c4c-13e7-486b-b073-fc64b81eeb4b
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/main.go

@@ -141,7 +141,7 @@ var rootCmd = &cobra.Command{
 		}
 
 		// Skip database initialization for commands that don't need a database
-		noDbCommands := []string{"init", cmdDaemon, "help", "version", "quickstart", "doctor"}
+		noDbCommands := []string{"init", cmdDaemon, "help", "version", "quickstart", "doctor", "merge"}
 		if slices.Contains(noDbCommands, cmd.Name()) {
 			return
 		}