refactor(tooling): replace ESLint + Prettier with Biome v2

[kylejryan]

Summary

Switches the lint/format toolchain from ESLint + Prettier to Biome v2.4.14 for JS/TS/JSX/TSX/JSON/CSS, with a slim Prettier install kept solely for Markdown and YAML (still in-progress in Biome v2 per its language support page). All ESLint rule choices and Prettier formatting defaults are preserved; behavior parity verified against the full CI suite.

Lines changed (excluding mechanical reformat)

Bucket	Files	LOC
Real changes (config, scripts, intentional source edits)	15	+523 / -530
Mechanical reformat (Biome's organizeImports import-sorting)	270	+1,305 / -1,339
Total	285	+1,828 / -1,869

The 270-file reformat commit is split out (commit `8f36d01b`) so reviewers can ignore it after a spot-check — every hunk in that commit is alphabetical import sorting, with no semantics changes.

Within the "real changes" bucket, ~157 of the 523 lines are `package.json` getting alphabetized by Biome's `useSortedKeys` (intended QoL), so true intent-only is closer to ~370 LOC of human-meaningful change.

Performance

End-to-end CI lint job timing on this dev machine, 3 warm runs each, median:

Step	Old (ESLint+Prettier)	New (Biome+slim Prettier)	Speedup
Lint `src/`	1.757s	0.131s	13.4×
Format check (full repo)	2.583s	0.576s	4.5×
Combined CI lint job	4.340s	0.707s	6.1×

Biome utilizes ~700% CPU (parallel across cores) vs ESLint/Prettier's ~200% — the gap widens further on bigger CI boxes. And the new path does strictly more in that 0.7s (lint + format + import-organize, vs old format-only).

Behavior preserved

All ESLint rule choices ported 1:1:

• `noExplicitAny`: error
• `noNamespace`: error
• `noUnusedImports`: error (was `unused-imports/no-unused-imports`)
• `noUnusedVariables`, `useExhaustiveDependencies`, `noCommonJs`, `noUselessEscape*`: off (carryover)

Prettier formatting defaults are preserved (2-space indent, double quotes, semicolons, trailing commas all, line width 80) — Biome v2 defaults match Prettier 3.x defaults.

Quality-of-life additions

• Single tool, single config (`biome.jsonc`)
• Built-in import organizer on save (`source.organizeImports.biome`)
• `useSortedKeys` on `package.json` keeps the manifest alphabetized
• `vcs.useIgnoreFile` — single source of truth for ignore patterns (`.gitignore`)
• Domain-aware lint rules (`react`, `test`) without third-party plugins
• New scripts: `bun run check` (unified lint+format+organize), `bun run lint:fix` (auto-fix that didn't exist before)

Documented deviations from a vanilla port

Each rationale is inline in biome.jsonc:

1. `useSortedKeys` scoped to `package.json` only via `overrides`. Globally enabling it scrambled `tsconfig.json`'s manually-grouped sections (3,300+ extra diagnostics).
2. `a11y` recommended set turned off. This is a TUI codebase using opentui's ``/`` terminal primitives, not DOM. The recommended a11y rules misfired on every interactive ``.
3. Triaged disables with rationale + "revisit in follow-up triage PR" notes for `noArrayIndexKey`, `noImplicitAnyLet`, `noAssignInExpressions`, `useIterableCallbackReturn`, `useHookAtTopLevel` (false-positive in `src/tui/keybindings/registry.ts`).
4. `mdx` removed from Prettier scope to preserve the old `.prettierignore` exclusion (Fern docs use MDX features Prettier can't parse).

Source-code fixes (not reformatting)

7 files have intentional non-formatting changes:

• Converted 5 `// eslint-disable-next-line` comments to `// biome-ignore lint/...` with rationale strings:
  • src/tui/auto-copy.ts
  • src/tui/console-theme.ts
  • src/core/agents/offSecAgent/offensiveSecurityAgent.ts
  • src/core/ai/streamIdleTimeout.test.ts
  • scripts/validate-shell.ts
• Removed two genuinely-unused imports surfaced by Biome's stricter `noUnusedImports`:
  • `ensureSandboxPlaywright` from scripts/test-sandbox-playwright.ts
  • default-`React` from src/tui/components/ascii-art.tsx

Test plan

• `bun run lint` — green
• `bun run format:check` — green
• `bun run tsc` — green (34s)
• `bun run test` — green (905 passed, 16 skipped, 26s)
• `bun run build` — green
• CI green on this PR
• VSCode Biome extension installed by reviewers, format-on-save verified

Follow-ups (deferred to a triage PR)

The 443 warnings + 321 infos surfaced by Biome's broader `recommended` ruleset are out of scope here. The disabled-with-rationale rules in `biome.jsonc` are tagged "revisit in a follow-up triage PR" so they show up via grep when that work is picked up.

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Note

Medium Risk
Switches the project’s lint/format pipeline and editor integration from ESLint/Prettier to Biome, which can cause CI failures or noisy diffs if rule/format parity isn’t exact. Runtime code behavior should be unaffected aside from any fixes driven by new lint diagnostics.

Overview
Migrates linting/formatting from ESLint + Prettier to Biome v2.4.14 for JS/TS/JSON/CSS, while keeping Prettier only for Markdown/YAML.

Updates the reusable CI lint workflow (.github/workflows/lint.yml) and VSCode defaults (.vscode/settings.json) to run/fix via Biome (including import organization), adds a new biome.jsonc with rule/formatter settings and package.json-only key-sorting, and refreshes AGENTS.md command/CI documentation. Removes the repo’s .prettierignore in favor of Biome’s ignore/include configuration.

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