ci: don't run the Android build for forge-housekeeping changes

The docs-only skip worked, but its skip-list was narrow enough that PR #96 —
a contributing guide, docs edits and issue templates — built the app four
times over two files that Gradle never reads: an issue-template config.yml
and .gitignore. Adds issue templates, .planning/, licences/, renovate.json5
and the dotfiles to the list, and logs which files forced a build so the
next surprise is answerable from the run log.

Also normalises a full refs/heads/ base_ref, which would otherwise fail the
merge-base lookup and silently turn the whole guard into "always build".

.forgejo/workflows/ci.yaml

@@ -37,14 +37,29 @@ jobs:
       - name: Reproducible-release invariant
         run: bash scripts/check_reproducible_release.sh
 
-      # Decide whether anything that affects the app build changed. Docs,
+      # Decide whether anything that affects the app build changed. Docs, store
-      # F-Droid metadata and the licence don't, so those PRs skip the SDK +
+      # metadata, licence texts and forge housekeeping don't, so those PRs skip
-      # Gradle work below but still report a green `ci`.
+      # the SDK + Gradle work below but still report a green `ci`.
       - name: Classify change scope
         id: scope
+        env:
+          # Deliberately a skip-list, not a build-list: a path nobody thought
+          # about defaults to building. Only paths the Gradle build provably
+          # never reads belong here — note that the workflows themselves, the
+          # `.gitmodules` submodule pointer and `scripts/` are *not* in it.
+          SKIP_RE: '(\.md$|^docs/|^fastlane/|^fdroid-metadata/|^licenses/|^\.planning/|^\.(forgejo|gitea)/ISSUE_TEMPLATE/|^\.editorconfig$|^\.gitattributes$|^\.gitignore$|^renovate\.json5$|^LICENSE$)'
         run: |
           set -e
           BASE="${{ github.base_ref }}"
+          # Normally the bare branch name; tolerate a full ref, which would
+          # otherwise make the merge-base lookup fail and quietly degrade this
+          # guard into "always build".
+          BASE="${BASE#refs/heads/}"
+          if [ -z "$BASE" ]; then
+            echo "No base branch on this event — running the full build to be safe."
+            echo "code=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
           # Full (not --depth=1) base fetch so the merge-base is present even when
           # the PR branch forked several commits back; a shallow tip has no merge
           # base with a divergent branch and `git diff base...HEAD` aborts.
@@ -58,11 +73,15 @@ jobs:
           fi
           CHANGED=$(git diff --name-only "$MB" HEAD)
           echo "Changed files:"; echo "$CHANGED"
-          if echo "$CHANGED" | grep -vE '(\.md$|^docs/|^fdroid-metadata/|^fastlane/|^LICENSE$)' | grep -q .; then
+          RELEVANT=$(echo "$CHANGED" | grep -vE "$SKIP_RE" || true)
+          if [ -n "$RELEVANT" ]; then
+            # Naming them makes "why did my docs PR build for four minutes?"
+            # answerable from the log alone.
+            echo "Build-relevant changes:"; echo "$RELEVANT"
             echo "code=true" >> "$GITHUB_OUTPUT"
           else
-            echo "code=false" >> "$GITHUB_OUTPUT"
             echo "Docs/metadata-only change — skipping the Android build."
+            echo "code=false" >> "$GITHUB_OUTPUT"
           fi
 
       - name: Setup Java