fix(ci): correct subpath URL emit + build into public/tsuki for htmltest

Run 25919953710 still failed htmltest on both Hugo legs. Two distinct
issues, both pre-existing latent bugs that v0.3.0 made impossible to
ignore:

1. Several theme templates piped paths with a leading slash through
   `relURL`, which in Hugo means "this is already absolute — leave it
   alone." The intended subpath prefix (`/tsuki/`) was therefore never
   added, producing hrefs like `href="/search/"` and `href="/"`. In
   production the GitHub Pages mount masked this (broken links from
   the page's perspective still happened to resolve under the repo
   path most of the time), but htmltest correctly reported them as
   broken. Fix by either:
   - using `site.Home.RelPermalink` for the home link, or
   - piping a string without a leading slash through `relLangURL`
     (so Hugo prepends the baseURL path), or
   - stripping the leading slash from data-driven URLs first via
     `strings.TrimPrefix "/"` and then `relLangURL`.

   Affected: header.html, search-button.html, recent-posts.html,
   home/hero.html, nav.html, 404.html, search/list.html (Pagefind UI
   asset URLs).

2. htmltest has no URL-rewriting feature (the `URLSwap` config in the
   prior `d30f50f` "fix" was never read by htmltest — confirmed by
   reading htmltest 0.17.0 source). The only clean way to make
   internal-link checking work with a `/repo/`-style baseURL is to
   mirror the URL structure on disk. CI now builds with
   `hugo --destination public/tsuki`, runs Pagefind/smoke/htmltest
   against that path, and uploads `exampleSite/public/tsuki/` as the
   Pages artifact. The artifact contents become the served site
   under `/tsuki/`, identical to the prior behaviour from the
   browser's point of view. `.htmltest.yml` drops the dead URLSwap
   block.

Also: `tmp/` (htmltest's runtime cache) added to .gitignore so local
runs don't dirty the working tree.

Local verification: 32 smoke checks green; htmltest reports
`✔✔✔ passed, tested 29 documents` against the subpath build.

.github/workflows/pages.yml

@@ -44,14 +44,18 @@ jobs:
         run: npm ci
 
       - name: Build site
+        # Build into public/tsuki so the on-disk layout mirrors the URL structure
+        # (`/tsuki/foo` href → exampleSite/public/tsuki/foo file). This lets
+        # htmltest resolve internal links without needing URL-rewriting (which
+        # htmltest does not support).
         working-directory: exampleSite
-        run: hugo --gc --minify --baseURL "https://tiennm99.github.io/tsuki/"
+        run: hugo --gc --minify --destination public/tsuki --baseURL "https://tiennm99.github.io/tsuki/"
 
       - name: Build Pagefind index
-        run: npx pagefind --site exampleSite/public
+        run: npx pagefind --site exampleSite/public/tsuki
 
       - name: Smoke tests (SEO + a11y + per-kind CSS budget + features regression guard)
-        run: ./scripts/smoke-tests.sh exampleSite/public
+        run: ./scripts/smoke-tests.sh exampleSite/public/tsuki
 
       - name: htmltest (broken internal links + HTML5 validation)
         # Pinned to master SHA (2026-05-10) for supply-chain hygiene. Refresh periodically.
@@ -60,11 +64,12 @@ jobs:
           config: .htmltest.yml
 
       - name: Upload artifact
-        # Only upload from the current Hugo version; the floor-version run is for compatibility check.
+        # Upload only the inner build (exampleSite/public/tsuki). Pages mounts the
+        # artifact root at /tsuki/, so the inner contents become the served site.
         if: matrix.hugo == '0.154.0'
         uses: actions/upload-pages-artifact@v5
         with:
-          path: exampleSite/public
+          path: exampleSite/public/tsuki
 
   deploy:
     needs: build