feat(diff): add sync-push source filter

[realityforge]

Summary

• add braid diff --sync-push-only to select sources configured with sync_push: true
• quietly skip ineligible explicit selections and avoid cache or Git work when no source qualifies
• preserve existing diff semantics and Git passthrough behavior
• document the option and cover parsing, help, completion, command behavior, and executable integration behavior

Validation

• bazel run @rules_go//go -- fmt ./...
• bazel test --test_env=PATH //...
• bazel run @rules_go//go -- vet ./...
• bazel run @rules_go//go -- run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.0 run
• bazel test --test_env=PATH //integration/...
• git diff --check