Move plugin version into .env, fix version-bump workflow to only touch it

The old workflow grepped CMakeLists.txt for the first "VERSION X.Y" match,
which was cmake_minimum_required's own version rather than the project's,
mangling it to 3.27.0.0 on the first real bump. PLUGIN_VERSION in .env is
now the single source of truth for both CMakeLists.txt and flake.nix, and
the workflow's bump/commit steps only ever touch that one file.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/version-bump.yml

@@ -50,29 +50,26 @@ jobs:
           echo "::error::merged PR into develop has no bump/major, bump/minor, bump/patch, or bump/skip label"
           exit 1
 
-      - name: Bump CMakeLists.txt and flake.nix
+      - name: Bump PLUGIN_VERSION in .env
         if: steps.level.outputs.level != 'skip'
         env:
           LEVEL: ${{ steps.level.outputs.level }}
         run: |
           set -euo pipefail
 
-          CURRENT=$(grep -oP '(?<=VERSION )[0-9]+(\.[0-9]+){1,2}' CMakeLists.txt | head -1)
-
-          # Pad to 3 components so a pre-existing 2-component version (e.g.
-          # "0.1") still splits into MAJOR/MINOR/PATCH correctly.
-          IFS='.' read -r MAJOR MINOR PATCH <<< "${CURRENT}.0.0"
+          CURRENT=$(sed -n 's/^PLUGIN_VERSION=//p' .env)
+          IFS='.' read -r MAJOR MINOR PATCH <<< "$CURRENT"
 
           case "$LEVEL" in
             major) MAJOR=$((MAJOR + 1)); MINOR=0; PATCH=0 ;;
             minor) MINOR=$((MINOR + 1)); PATCH=0 ;;
             patch) PATCH=$((PATCH + 1)) ;;
+            *) echo "::error::unknown bump level '$LEVEL'"; exit 1 ;;
           esac
 
           NEW="$MAJOR.$MINOR.$PATCH"
 
-          sed -i "s/VERSION ${CURRENT}/VERSION ${NEW}/" CMakeLists.txt
-          sed -i "s/version = \"${CURRENT}\"/version = \"${NEW}\"/" flake.nix
+          sed -i "s/^PLUGIN_VERSION=.*/PLUGIN_VERSION=${NEW}/" .env
 
           echo "CURRENT_VERSION=${CURRENT}" >> "$GITHUB_ENV"
           echo "NEW_VERSION=${NEW}" >> "$GITHUB_ENV"
@@ -87,6 +84,6 @@ jobs:
           fi
           git config user.name "gitea-actions"
           git config user.email "actions@noreply.localhost"
-          git add CMakeLists.txt flake.nix
-          git commit -m "Bump version to ${NEW_VERSION}"
+          git add .env
+          git commit -m "Bump PLUGIN_VERSION to ${NEW_VERSION}"
           git push origin HEAD:develop