chore: Convert formatter from Biome + Prettier to Oxfmt

[roottool]

Description

Replaces the two-formatter setup (Biome for TS/JS + Prettier for MD/YAML) with a single formatter, Oxfmt, which handles all file types. This simplifies the toolchain and removes the need to coordinate between two tools.

Type of Change

• Bug fix (non-breaking change fixing an issue)
• Security fix (non-breaking change addressing a security issue)
• Documentation (changes to documentation only)
• Refactoring (non-breaking change improving code structure)
• Tests (adding or updating tests)
• Tooling / CI (changes to build tools, CI configuration)

Boundary Checklist (Required for Implementation Changes)

• This is a non-implementation change (Documentation, Tooling, or CI only)
  • Reason: This PR only changes the formatter toolchain. No changes to parser logic, public API, or security rules.

Security & API Stability

Security Impact:

• No impact on security (no changes to forbidden keys, prototype safety)
• Reviewed against security rules in AGENTS.md
• If security-related: Explain impact and mitigation

API Contract:

• No changes to public API (parse function signature, type definitions)
• No breaking changes to ParseResult, ParseIssue, or IssueCode
• If API change: Is this a breaking change? (Yes/No + explanation)

Versioning:

• Change is compatible with current v0.x versioning policy
• If breaking change proposed: Justification for major version bump

Testing

Automated Checks:

• TypeScript type checking passes (bun run check:type:source)
• All tests pass (bun run test)
• Build succeeds (bun run build)

Additional Verification:

All existing checks continue to pass with the new formatter. The check:format script now delegates to oxfmt --check instead of running biome and prettier sequentially. The CI workflow needs array was collapsed to a single line to match Oxfmt's formatting style.

Summary of Changes

• Removed: biome.json, Biome and Prettier dev dependencies
• Added: .oxfmtrc.json Oxfmt configuration
• Updated: package.json — simplified format, check:format, and check:source:ci scripts
• Updated: .github/workflows/ci.yml — collapsed needs array to one line (Oxfmt style)
• Updated: README.md — added Contributing section
• Updated: CLAUDE.md, skill reference docs — replaced Prettier/Biome mentions with Oxfmt
• Applied: Oxfmt formatting across all tracked files

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🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (76042ae) to head (d9ef05f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #54   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines           22        22           
  Branches         6         6           
=========================================
  Hits            22        22           

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