feat(format): add oxfmt formatter support

[chrisolszewski]

Summary

Adds oxfmt as an optional formatter for TypeScript and JavaScript. It's
auto-detected from its own config files and can be set explicitly with
formatter.typescript: "oxfmt". Biome is still the first formatter
auto-detection looks for; oxfmt applies only when there's no Biome config.

What changed

• Rust (format.rs, configure.rs): detect .oxfmtrc.json, .oxfmtrc.jsonc,
  and oxfmt.config.ts; run oxfmt --write <file>; add an install hint; teach
  formatter_excluded_path to recognize oxfmt's "no files" messages so an
  ignored path reports format_skipped_reason: formatter_excluded_path instead
  of an error.
• Config and schema: add oxfmt to the formatter enum in the opencode and pi
  plugins, assets/aft.schema.json, and the schema generator.
• Docs: README and the pi README list oxfmt next to biome and prettier, biome
  first.
• Tests: detection, explicit override, excluded-path handling, integration write.

Why biome stays first

Auto-detection order doesn't change. oxfmt is picked only when no biome config
is present, so existing biome projects behave exactly as before.

Validation

• cargo fmt --check; cargo test -p agent-file-tools format
• bun test for the opencode and pi config tests
• bun run typecheck in both plugins

---

Summary by cubic

Adds optional oxfmt formatting for TypeScript/JavaScript. It’s auto-detected from its config, stays behind biome in priority, and can be set with formatter.typescript: "oxfmt". No changes for existing biome projects.

• New Features
  • Auto-detects .oxfmtrc.json, .oxfmtrc.jsonc, and oxfmt.config.ts; runs oxfmt --write <file>.
  • Supports formatter.typescript: "oxfmt" in config; schemas updated in both plugins.
  • Finds local oxfmt in node_modules/.bin and shows an install hint when missing.
  • Maps oxfmt “no files” errors to format_skipped_reason: formatter_excluded_path.

^{Written for commit c505a41. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 12 files

_{Re-trigger cubic}