feat: add ESLint, lint-staged, and update pre-commit hook + CI

- Add ESLint 9 flat config (eslint.config.mjs) with typescript-eslint
  recommended rules and eslint-config-prettier
- Add lint-staged to run eslint+prettier only on staged files
- Update pre-commit hook to use lint-staged instead of full prettier check
- Add `lint` and `format:check` scripts to package.json
- Add Lint step to CI workflow
- Fix resulting lint errors: unused vars (_ctx, _options, catch binding),
  any→unknown in type declarations, stale eslint-disable comments

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

src/types/index.ts

@@ -71,7 +71,7 @@ declare global {
   // This is not an ideal solution but works for our example
   interface KVNamespace {
     get(key: string, options?: { type: "text" }): Promise<string | null>;
-    get(key: string, options: { type: "json" }): Promise<any | null>;
+    get(key: string, options: { type: "json" }): Promise<unknown>;
     get(
       key: string,
       options: { type: "arrayBuffer" },

src/types/rss.d.ts

@@ -15,7 +15,7 @@ declare module "rss" {
     generator?: string;
     categories?: string[];
     custom_namespaces?: Record<string, string>;
-    custom_elements?: any[];
+    custom_elements?: unknown[];
   }
 
   interface RSSItemOptions {
@@ -34,7 +34,7 @@ declare module "rss" {
       size?: number;
       type?: string;
     };
-    custom_elements?: any[];
+    custom_elements?: unknown[];
   }
 
   interface RSSXMLOptions {