chore(tooling): migrate to Ultracite + oxfmt

[kattsushi]

What changed

• Switched formatting from dprint to Oxfmt and standardized the workflow via Ultracite (Oxlint + Oxfmt).
• Added repo-level configs: oxfmt.config.ts, oxlint.config.ts.
• Updated CI to use the single gate: pnpm run check.
• Updated lint-staged and VSCode workspace settings/extensions for the new formatter.
• Added Nx shared agent standards under .agents/ only (removed other per-agent outputs).

How to use

• Check: pnpm run check
• Fix: pnpm run fix

Verification

• pnpm run check
• pnpm run typecheck
• pnpm run build

Notes / limitations

• Oxfmt cannot enforce multiline/indented ternaries; short ternaries may be collapsed when they fit within printWidth.
• .atl/skill-registry.md is intentionally not committed (local index/cache per gentle-ai policy).