Add CI lint regression check via cross-ref ESLint diff

[gtbuchanan]

Problem

eslint-plugin-diff catches violations on changed lines, but can't detect regressions introduced at a distance (e.g., renaming an export breaks lint in untouched files). A cross-ref diff of lint results can catch these.

Implementation

Formatter (CLI package)

• Custom ESLint formatter that writes slim JSON to dist/eslint-results.json (files with violations only, fields: filePath, ruleId, line, column, endLine, endColumn, severity, message)
• Returns stylish formatter output for console display
• Used by lint:eslint via --format flag

Compare command (CLI package, not a turbo task)

• gtb lint:eslint:compare --base-sha <sha>
• Reads dist/eslint-results-base.json and dist/eslint-results.json per package
• Computes git diff <base-sha> HEAD to build line offset mappings
• Adjusts base violation positions using the mappings
• Outputs new violations (violations in HEAD not present in adjusted base)
• Exits non-zero if any new violations found

Workflow (new or added to existing CI)

• Triggers on push to non-main branches + pull requests
• Computes merge base against main
• Checks out base SHA, runs turbo run lint:eslint (remote cache hit)
• Copies each package's dist/eslint-results.json to dist/eslint-results-base.json
• Checks out HEAD, runs turbo run lint:eslint (from existing build, or reruns affected)
• Runs gtb lint:eslint:compare --base-sha <merge-base-sha>
• Posts/updates a single PR comment with grouped violations (hidden HTML marker for idempotent updates)

Gating

• Blocking for all PRs (human and bot)
• PR label override to accept known regressions (e.g., dependency upgrades introducing new rules)
• On push without PR: commit status. Failure triggers Renovate PR creation for bot branches

Prerequisites

• Add eslint-plugin-diff to pre-commit/prek ESLint hook (diff-only local linting)
• Add dist/eslint-results.json to turbo outputs for lint:eslint
• Wire --format into lint:eslint CLI command