chore: migrate lint and format from ESLint+Prettier to oxlint+oxfmt

[martyy-code]

Mirrors the migration landed in deessejs/fp PR #444. Replaces the ESLint flat configs in packages/errors and apps/web and the root .prettierrc with a single oxlint + oxfmt setup.

What

• Add oxlint@^1.87 and oxfmt@^0.72 as root dev dependencies.
• Remove eslint, @eslint/js, typescript-eslint, and eslint-config-next.
• New root configs: .oxlintrc.json and .oxfmtrc.json (the latter migrated from .prettierrc).
• Per-workspace configs at packages/errors/.oxlintrc.json and apps/web/.oxlintrc.json. A local override at apps/web/tests/seo/fixtures/.oxlintrc.json disables eslint/no-await-in-loop for that test fixture (the sequential awaits are intentional).
• Scripts: root format and format:check now use oxfmt; per-workspace lint and lint:fix use oxlint. lint-staged invokes oxfmt and oxlint --fix.
• Husky pre-commit now runs pnpm exec lint-staged && pnpm format:check && pnpm turbo type-check so format drift is caught on commit, not in CI.

Why

oxlint is a Rust-based linter that is materially faster than ESLint (single-digit seconds across the full workspace). oxfmt is its companion formatter. Together they replace the ESLint + Prettier pair with a single toolchain, drop the typescript-eslint plugin, and remove the legacy eslint-config-next that the web app was depending on without its own lint script. The migration also surfaces a class of issues that the prior ESLint config did not flag (e.g. consistent-type-imports, no-await-in-loop).

Source fixes driven by the migration

Four files were touched to satisfy rules that did not exist in the prior ESLint config:

• packages/errors/tests/edge-cases.test.ts: inline import('@standard-schema/spec').StandardSchemaV1 annotation replaced with a proper type import.
• apps/web/src/app/sitemap.ts: two duplicate imports from @/lib/source merged.
• apps/web/proxy.ts: NextRequest (type) split from NextResponse (value) to mark the type-only import.
• apps/web/tests/seo/fixtures/stubs/fumadocs.ts: left as-is; the await in loop is intentional and the rule is disabled in the local override.

Validation

• pnpm lint — 0 errors, 4 warnings (non-blocking; the same warning classes that deessejs/fp landed with)
• pnpm format:check — clean
• pnpm type-check — clean, both packages
• pnpm test — all tests pass
• pnpm build — clean, both packages

Notes

• This is a developer-tooling change only. The published @deessejs/errors artifact, the public API, and the runtime behavior are unchanged.
• oxfmt reformatted the existing source files to match its style. The diff is large (~190 files) but mechanical: line endings and trailing commas are the main visible changes. A separate formatting-only commit is not produced; the reformat is folded into the migration commit so the working tree ends in a clean state.
• The pre-commit hook now runs three commands sequentially (lint-staged, format:check, type-check). On a clean working tree the total pre-commit time is ~5s.

[martyy-code]

Superseded by #97 (the original branch was hitting Internal Server Error on multiple push attempts, so a fresh branch was created with the same content). The remaining CI gap on this PR is the same as on #97: staging does not yet have the unified ci.yml from PR #92 (which is only on main). Until the consolidated ci.yml lands on staging, only the changeset-check job is visible — the full 8-job matrix (Build, Lint, Tests × 3, Type Check × 3) won't run.