Drive lint:eslint to green and enforce in CI (decision: keep ESLint)

[mherod]

Context

While adding the node:fs import ban to ESLint (PR #551, mirroring the Biome rule from #550), it surfaced that pnpm lint:eslint is not an enforced gate:

• pnpm lint:eslint currently reports 543 errors / 54 warnings across the codebase (complexity, max-depth, jsdoc/*, @typescript-eslint/no-unnecessary-condition, etc.).
• It is not wired into CI or the husky pre-commit/pre-push hooks — pnpm lint (Biome) is the sole enforced linter, used in validate and the git hooks.

As a result, the ESLint copy of the node:fs ban (and every other ESLint rule) is currently defense-in-depth only — it never actually blocks a commit, push, or merge.

Decision needed

• (a) Fix & enforce — progressively drive lint:eslint to green and add it as a required CI gate, so the stricter TS-aware rules (complexity caps, jsdoc completeness, type-only import bans) are actually enforced.
• (b) Retire — formally remove lint:eslint / lint:all and keep Biome as the single source of truth. The node:fs ban already exists in Biome (chore(lint): ban direct node:fs imports in browser/cli modules #550), so no coverage is lost on that specific rule.

Whichever direction is chosen should be documented in CLAUDE.md so future sessions don't re-discover the gap.

References

• node:fs ban in Biome: chore(lint): ban direct node:fs imports in browser/cli modules #550
• node:fs ban mirrored to ESLint: chore(lint): mirror node:fs import ban in ESLint config #551
• FileSystemAdapter runtime layer: refactor(browsers): route detection fs access through runtime adapter #548, refactor(browsers): route browser fs access via FileSystemAdapter #549

[mherod]

Decision: keep lint:eslint (maintainer call).

Option (b) — retire — is rejected. ESLint stays as the secondary linter. The remaining work this issue now tracks is option (a): progressively fix the 543 pre-existing errors and warnings, then add pnpm lint:eslint as an enforced CI gate so the stricter TS-aware rules (complexity caps, jsdoc completeness, type-only no-restricted-imports) actually block regressions.

No code is removed; the node:fs ban added in #551 stays active in both linters.