chore(biome): upgrade to v2

[BenSheridanEdwards]

Why

The npm registry reports @biomejs/biome@2.5.1 as latest, while the template was still on Biome 1.9.4. This upgrades the lint/format/import owner to the current major and keeps the repo's one-tool lint/format contract intact.

What Changed

• Upgraded @biomejs/biome from ^1.9.4 to ^2.5.1.
• Ran pnpm exec biome migrate --write, migrating biome.json to the 2.5.1 schema.
• Applied Biome 2's required safe fixes: import ordering and CSS formatting.
• Fixed new Biome 2 HTML/a11y diagnostics in team-walkthrough.html by adding SVG titles and tightening a selector.

Proof

• pnpm view @biomejs/biome version dist-tags --json confirmed latest is 2.5.1.
• pnpm exec biome migrate --write
• pnpm check — Biome 2 checked 28 files.
• pnpm verify
• pnpm e2e
• Visual proof attached or linked, or N/A with reason: N/A — no product UI/CSS behaviour changed; formatting-only CSS wrap plus static walkthrough SVG titles.
• StyleProof reviewed, or N/A with reason: latest-sha StyleProof passed with No visual changes.
• Other proof: pre-commit hook passed typecheck, staged Biome 2, Fallow, and gitleaks; pre-push hook passed coverage, build, and E2E.

CI Status

• Latest-sha GitHub Actions are green.
• Pending checks are named below.
• Failing checks are named below with the fix plan.

Latest-sha checks are green: CI quality, Fallow, unit/component tests, production build, E2E behaviour map, security audit, CodeQL, Lighthouse, computed-style visual gate, and StyleProof.

Risk And Rollback

Moderate tooling risk, low runtime risk. This changes the formatter/linter major version and its config schema. Rollback is reverting the single commit if CI or downstream formatting expectations reveal an issue.

Definition Of Done

• Behaviour is covered by the E2E map when user-visible behaviour changed. (N/A — no product behaviour changed.)
• Logic is unit-tested where logic changed. (N/A — no runtime logic changed.)
• Accessibility was considered and verified for interactive/UI changes. Biome 2 surfaced missing SVG titles in team-walkthrough.html; fixed and pnpm check passed.
• The diff follows .agents/project/ARCHITECTURE.md and CONVENTIONS.md.
• Any new architectural decision is recorded as an ADR. (N/A — same tool owner, newer major.)
• No gates were bypassed or weakened.

[github-actions]

Fallow audit report

Found 1 finding.

Details

Severity	Rule	Location	Description
minor	fallow/unused-dev-dependency	package.json:44	Package '@lhci/cli' is in devDependencies but never imported

Generated by fallow.

[github-actions]

Fallow audit report

0 inline findings selected for GitHub review.

[github-actions]

🗺️ StyleProof report

✓ No visual changes detected.