docs(changelog): add full CHANGELOG.md at repo root

Split the generated markdown into two files:

  - CHANGELOG.md at repo root — full history with the Keep a Changelog
    header. Human-facing, discoverable from the GitHub repo page.
  - update-json/changelog.md — still truncated to the last 5 versions,
    for the Magisk/KSU update popup.

Both are regenerated from changelog.json on every changelog-add.py
and update-version.py run.

Also switch the CI release-notes extraction to read CHANGELOG.md so
the body is future-proof once a tag ages out of the short popup file.

.github/workflows/ci.yml

@@ -207,11 +207,11 @@ jobs:
           path: dist/
           merge-multiple: true
 
-      - name: Extract release notes from changelog
+      - name: Extract release notes from CHANGELOG.md
         run: |
           TAG="${{ github.ref_name }}"
           awk -v t="^## ${TAG}\$" '$0~t{flag=1;next} /^## v/{flag=0} flag' \
-            update-json/changelog.md > release-notes.md
+            CHANGELOG.md > release-notes.md
           echo "=== release-notes.md ==="
           cat release-notes.md