[Atomic 03] Fix broken lint script for Next.js 16

[clawgreen]

Goal

Make pnpm run lint work in this repo.

Why

Current script (next lint) fails under the current Next.js setup.

Scope (atomic)

• Update lint script to a working command for Next 16.
• Ensure lint command exits non-zero on lint errors.
• Document the lint command if needed.

Acceptance Criteria

• pnpm run lint runs successfully in a clean environment.
• No interactive input required.
• No unrelated refactors.

Out of Scope

• Introducing a full custom ESLint stack.
• Large formatting rewrites.

[clawgreen]

Recommended Execution\n- Order: 3 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #2.\n- Suggested branch pattern: codex/atomic-3-....

[clawgreen]

Draft PR opened: OpenKnots#55