ci: build gstack with npm instead of bun

gstack moved to npm on 2026-08-17 and ships package-lock.json with no
bun.lock, but the Pages workflow still installed bun and ran
"bun install --frozen-lockfile". Builds stayed green only because bun
falls back to consuming package-lock.json, so the mismatch never
surfaced. Its six sibling subprojects already use npm ci.

Verified locally: npm ci && npm run build produces the same 4 pages plus
the sitemap.

.github/workflows/deploy-pages.yml

@@ -24,10 +24,6 @@ jobs:
         with:
           node-version: '24'
 
-      - uses: oven-sh/setup-bun@v2
-        with:
-          bun-version: 1.3.13
-
       - name: Build claudekit
         run: npm ci && npm test && npm run build
         working-directory: claudekit
@@ -41,7 +37,7 @@ jobs:
         working-directory: bmad
 
       - name: Build gstack
-        run: bun install --frozen-lockfile && bun run build
+        run: npm ci && npm run build
         working-directory: gstack
         env:
           SITE_URL: https://tiennm99dev.github.io