chore: migrate to tsuki theme

- Replace the hugo-theme-stack submodule with the tsuki theme vendored in
  themes/tsuki; drop .gitmodules
- config/_default/hugo.yml: theme: tsuki
- config/_default/permalinks.yml: keep /p/:slug/ for posts to preserve
  existing URLs (matches Stack default)
- config/_default/markup.yml: add parser.autoHeadingIDType: github-ascii
  for ASCII heading slugs
- config/_default/params.yml: add tsuki-required keys (search.enable,
  home.recentPostsCount); restructure comments.giscus to tsuki schema
  (repoId/categoryId lowercase, theme, lang, etc.). Stack-only params
  kept for now in case of rollback (silently ignored by tsuki).
- Move content/page/archives -> content/archives (tsuki uses kind=section)
- Move content/page/search -> content/search (drop json output;
  Pagefind index built in CI replaces Fuse.js search)
- Add data/profile.yaml driving the homepage hero
- Copy assets/img/avatar.png -> static/img/ for tsuki's relURL resolution

Verified: 717 pages build clean, 280 aliases preserve old URLs.
Posts at /p/<slug>/, comments + TOC + archives + search all render.

data/profile.yaml

@@ -0,0 +1,14 @@
+name: "miti99"
+handle: "tiennm99"
+tagline: "A developer wants to be magician."
+avatar: "img/avatar.png"
+bio: |
+  Vietnamese developer. Writing about code, work, and the small things in between.
+
+links:
+  - icon: github
+    label: GitHub
+    url: https://github.com/tiennm99
+  - icon: rss
+    label: RSS
+    url: /index.xml