chore: introduce linter/formatter (Biome or ESLint)

[luojiyin1987]

Context

The project has 14k+ AST nodes across 838 files with no automated code-style enforcement. Root devDependencies contains only typescript + @types/node. No ESLint, Prettier, or Biome is configured. All formatting and linting is manual.

Problem

• No consistent code style across contributors
• No automated enforcement of conventions (e.g. no console.log in production, consistent import ordering)
• PR reviews waste time on style nits
• CI pipeline (planned in chore: setup CI/CD pipeline with quality gates #414) has no lint gate to plug into

Proposal

Choose and configure one of:

Option A: Biome (recommended)

• Single binary, fast, covers both lint + format
• Built-in TypeScript/React rules
• Migration tool for ESLint configs

Option B: ESLint + Prettier

• Industry standard, larger plugin ecosystem
• More configuration overhead, slower

Implementation steps

1. Team decides on tool (Biome vs ESLint)
2. Add to root devDependencies
3. Configure ruleset (start with recommended, tune iteratively)
4. Add lint and format scripts to root package.json
5. Auto-fix existing violations in a dedicated PR
6. Add lint gate to CI (chore: setup CI/CD pipeline with quality gates #414)

Acceptance criteria

• Tool selected and configured
• npm run lint checks all workspaces
• npm run format auto-formats all workspaces
• Root package.json has lint and format scripts
• CI (chore: setup CI/CD pipeline with quality gates #414) includes lint gate

References

• chore: setup CI/CD pipeline with quality gates #414 — CI pipeline depends on this for the lint gate

[UncertaintyDeterminesYou4ndMe]

Claiming this. Plan per the issue's own sequencing: this first PR introduces Biome (Option A) with root lint/format scripts and a CI lint gate, with the ruleset tuned so the current tree passes (relaxed rules documented for follow-up); the full-repo auto-fix lands in a dedicated follow-up PR to keep diffs reviewable and avoid conflicting with the ~11 open PRs.