quine branch: README and the rebase workflow

The fork's default branch, with no KDE code: how the quine-v<version>
branches work, and a daily workflow that carries our commits onto each
new Plasma 6 release as a new quine-v<version> branch, failing if a
commit doesn't apply cleanly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.forgejo/workflows/rebase.yml

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+# Carries our commits onto each new Plasma release: for every Plasma 6
+# release tag newer than the newest quine-v* branch, makes quine-v<version>
+# = the tag + our commits from that branch. Fails, making no branch, if a
+# commit doesn't apply cleanly. See README.md.
+name: rebase
+
+on:
+  schedule:
+    # Between our Aurora build (05:00 UTC) and quine-linux's (07:00 UTC).
+    - cron: '0 6 * * *'
+  workflow_dispatch:
+
+concurrency:
+  group: rebase
+  cancel-in-progress: false
+
+jobs:
+  rebase:
+    runs-on: runner-x86-linux
+    env:
+      FORK: ${{ github.server_url }}/${{ github.repository }}.git
+      MIRROR: ${{ github.server_url }}/quine-global-labs/mirror-kde-plasma-desktop.git
+    steps:
+      - name: Make branches for new Plasma releases
+        env:
+          TOKEN: ${{ github.token }}
+        run: |
+          set -euo pipefail
+          git init -q work && cd work
+          git config user.name "Quine Linux CI"
+          git config user.email "ci@quinefoundation.com"
+          # The job token can push to this repo. Blob-less fetches: git
+          # fetches file contents from these remotes as cherry-picks need them.
+          git remote add origin "${FORK/https:\/\//https://ci:$TOKEN@}"
+          git remote add upstream "$MIRROR"
+          git fetch -q --filter=blob:none origin 'refs/heads/quine-v*:refs/remotes/origin/quine-v*'
+          git fetch -q --filter=blob:none upstream 'refs/tags/v6.*:refs/tags/v6.*'
+
+          # Newest branch we have, and release tags newer than it. Betas
+          # are x.y.80 and up.
+          newest=$(git for-each-ref --format='%(refname:lstrip=3)' 'refs/remotes/origin/quine-v*' |
+            sed 's/^quine-v//' | sort -V | tail -1)
+          [ -n "$newest" ] || { echo "no quine-v* branch to start from" >&2; exit 1; }
+          echo "newest branch: quine-v$newest"
+          new=$(git tag -l 'v6.*' | sed 's/^v//' |
+            grep -E '^6\.[0-9]+\.[0-9]+$' | awk -F. '$3 < 80' |
+            sort -V | awk -v n="$newest" '
+              BEGIN { split(n, b, ".") }
+              { split($0, a, ".")
+                if (a[1] > b[1] || (a[1] == b[1] && (a[2] > b[2] || (a[2] == b[2] && a[3] > b[3])))) print }')
+          if [ -z "$new" ]; then
+            echo "no Plasma release newer than $newest"
+            exit 0
+          fi
+
+          base=$newest
+          for v in $new; do
+            echo "::group::quine-v$v"
+            git switch -q --detach "v$v"
+            commits=$(git rev-list --reverse "v$base..origin/quine-v$base")
+            for c in $commits; do
+              if ! git cherry-pick -x "$c" >/dev/null; then
+                git cherry-pick --abort || true
+                echo "::error::$(git log -1 --format='%h %s' "$c") doesn't apply to v$v; make quine-v$v by hand (README.md)"
+                exit 1
+              fi
+            done
+            git push -q origin "HEAD:refs/heads/quine-v$v" "refs/tags/v$v"
+            git update-ref "refs/remotes/origin/quine-v$v" HEAD
+            echo "made quine-v$v: v$v + $(echo "$commits" | wc -w) commit(s) from quine-v$base"
+            echo "::endgroup::"
+            base=$v
+          done

README.md

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+# plasma-desktop, Quine Linux's fork
+
+KDE's [plasma-desktop](https://invent.kde.org/plasma/plasma-desktop), with
+the changes Quine Linux ships. Copied from our pull mirror,
+[`mirror-kde-plasma-desktop`](../mirror-kde-plasma-desktop).
+
+## Branches
+
+- **`quine-v<version>`**: KDE's release tag `v<version>` plus our commits.
+  [quine-linux](../quine-linux) builds Kickoff (the start menu) from the
+  branch for the Plasma version its image ships (its Containerfile's
+  `kickoff-builder` stage); there's one per Plasma release since 6.7.5.
+- **`quine`** (default): this README and the workflow below. No KDE code.
+- **`master`** and KDE's tags: as copied from upstream.
+
+Our commits so far:
+
+- Kickoff: an update bar ("A system update is available" / "Restart to
+  finish updating"), fed by quine-linux's `quine-update-check.service`.
+
+## Following new Plasma releases
+
+[`.forgejo/workflows/rebase.yml`](.forgejo/workflows/rebase.yml) runs daily
+at 06:00 UTC, between our Aurora build (05:00) and quine-linux's (07:00).
+For every Plasma 6 release tag newer than the newest `quine-v*` branch (from
+the mirror; betas, `.80` and up, are skipped), it puts our commits from that
+branch on top of the tag as a new `quine-v<version>` branch.
+
+If a commit doesn't apply cleanly, the run fails and makes no branch. Then
+make the branch by hand:
+
+```sh
+git fetch origin && git fetch https://forge.quinefoundation.com/quine-global-labs/mirror-kde-plasma-desktop.git 'refs/tags/v<new>:refs/tags/v<new>'
+git switch -c quine-v<new> v<new>
+git cherry-pick v<old>..origin/quine-v<old>   # resolve, git cherry-pick --continue
+git push origin quine-v<new> v<new>
+```
+
+Until then, quine-linux's build fails when its image moves to that Plasma
+version, saying which branch is missing.
+
+To change our commits: commit on the newest `quine-v*` branch (and older
+ones still in use, if needed); the next release's branch picks them up.