perf: add timeout and watch-mode cache extension to all data files

Introduce shared cachedFetch helper (lib/data-fetch.js) wrapping
EleventyFetch with two protections:

- 10-second hard timeout via AbortController on every network request,
  preventing slow or unresponsive APIs from hanging the build
- 4-hour cache TTL in watch/serve mode (vs 5-15 min originals), so
  incremental rebuilds serve from disk cache instead of re-fetching
  APIs every time a markdown file changes

All 13 network _data files updated to use cachedFetch. Production
builds keep original short TTLs for fresh data.

Targets the "Data File" benchmark (12,169ms / 32% of incremental
rebuild) — the largest remaining bottleneck after filter memoization.

Confab-Link: http://localhost:8080/sessions/0b241cd6-aff2-4fec-853c-2b5a61e61946

_data/githubRepos.js

@@ -3,7 +3,7 @@
  * Fetches public repositories from GitHub API
  */
 
-import EleventyFetch from "@11ty/eleventy-fetch";
+import { cachedFetch } from "../lib/data-fetch.js";
 
 export default async function () {
   const username = process.env.GITHUB_USERNAME || "";
@@ -12,7 +12,7 @@ export default async function () {
     // Fetch public repos, sorted by updated date
     const url = `https://api.github.com/users/${username}/repos?sort=updated&per_page=10&type=owner`;
 
-    const repos = await EleventyFetch(url, {
+    const repos = await cachedFetch(url, {
       duration: "1h", // Cache for 1 hour
       type: "json",
       fetchOptions: {