Add an ESLint config for the extension and wire it into CI

[AshwinUgale]

Why

extension/package.json has a placeholder lint script ("lint": "echo 'eslint placeholder'") and CI runs ESLint only if .eslintrc.cjs exists. Adding a real config turns on linting for every future PR at zero extra CI wiring - the guarded step is already in .github/workflows/ci.yml.

What to do

• Add ESLint + @typescript-eslint as devDeps, add a .eslintrc.cjs (or flat eslint.config.js) with a sensible TS baseline, and replace the placeholder lint script with a real eslint src.
• Fix any warnings so eslint src --max-warnings=0 passes.

Acceptance criteria

• npx tsc --noEmit still clean; npm run lint clean.
• The existing CI ESLint step now runs and passes.

Pointers

• extension/package.json scripts.lint; the ESLint step in .github/workflows/ci.yml.