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>

eslint.config.js

@@ -0,0 +1,19 @@
+import eslint from "@eslint/js";
+import tseslint from "typescript-eslint";
+
+export default tseslint.config(
+  eslint.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    files: ["src/**/*.ts"],
+    rules: {
+      "@typescript-eslint/no-explicit-any": "off",
+      "@typescript-eslint/no-unused-vars": ["warn", { argsIgnorePattern: "^_" }],
+      "@typescript-eslint/no-unsafe-function-type": "off",
+      "preserve-caught-error": "off",
+    },
+  },
+  {
+    ignores: ["dist/", "node_modules/", "**/*.js", "!eslint.config.js"],
+  },
+);