fix(website): run skill extraction automatically on npm run build/start (#13747)

website/src/pages/skills/index.tsx imports ../../data/skills.json, but
that file is git-ignored and generated at build time by
website/scripts/extract-skills.py. CI workflows (deploy-site.yml,
docs-site-checks.yml) run the script explicitly before 'npm run build',
so production and PR checks always work — but 'npm run build' on a
contributor's machine fails with:

  Module not found: Can't resolve '../../data/skills.json'

because the extraction step was never wired into the npm scripts.

Adds a prebuild/prestart hook that runs extract-skills.py automatically.
If python3 or pyyaml aren't installed locally, writes an empty
skills.json instead of hard-failing — the Skills Hub page renders with
an empty state, the rest of the site builds normally, and CI (which
always has the deps) still generates the full catalog for production.

website/scripts/prebuild.mjs

@@ -0,0 +1,50 @@
+#!/usr/bin/env node
+// Runs website/scripts/extract-skills.py before docusaurus build/start so
+// that website/src/data/skills.json (imported by src/pages/skills/index.tsx)
+// exists without contributors needing to remember to run the Python script
+// manually. CI workflows still run the extraction explicitly, which is a
+// no-op duplicate but matches their historical behaviour.
+//
+// If python3 or its deps (pyyaml) aren't available on the local machine, we
+// fall back to writing an empty skills.json so `npm run build` still
+// succeeds — the Skills Hub page just shows an empty state. CI always has
+// the deps installed, so production deploys get real data.
+
+import { spawnSync } from "node:child_process";
+import { mkdirSync, writeFileSync, existsSync } from "node:fs";
+import { dirname, join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const scriptDir = dirname(fileURLToPath(import.meta.url));
+const websiteDir = resolve(scriptDir, "..");
+const extractScript = join(scriptDir, "extract-skills.py");
+const outputFile = join(websiteDir, "src", "data", "skills.json");
+
+function writeEmptyFallback(reason) {
+  mkdirSync(dirname(outputFile), { recursive: true });
+  writeFileSync(outputFile, "[]\n");
+  console.warn(
+    `[prebuild] extract-skills.py skipped (${reason}); wrote empty skills.json. ` +
+      `Install python3 + pyyaml locally for a populated Skills Hub page.`,
+  );
+}
+
+if (!existsSync(extractScript)) {
+  writeEmptyFallback("extract script missing");
+  process.exit(0);
+}
+
+const result = spawnSync("python3", [extractScript], {
+  stdio: "inherit",
+  cwd: websiteDir,
+});
+
+if (result.error && result.error.code === "ENOENT") {
+  writeEmptyFallback("python3 not found");
+  process.exit(0);
+}
+
+if (result.status !== 0) {
+  writeEmptyFallback(`extract-skills.py exited with status ${result.status}`);
+  process.exit(0);
+}