chore: add ESLint with TypeScript support and baseline config

Install eslint, @eslint/js, and typescript-eslint as dev dependencies.
Add eslint.config.js (flat config) with rules tuned to zero errors on
the current codebase, add ESLint local hook to .pre-commit-config.yaml,
and update the lint:ts npm script to use ESLint directly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.pre-commit-config.yaml

@@ -44,3 +44,12 @@ repos:
         args: [--config-file=pyproject.toml]
         pass_filenames: false
         additional_dependencies: []
+
+  - repo: local
+    hooks:
+      - id: eslint
+        name: eslint
+        entry: npx eslint --fix
+        language: node
+        types: [ts]
+        files: ^src/