fix(ci): use go-test-coverage binary directly to fix Gitea Actions (#150)

## Summary

- `vladopajic/go-test-coverage@v2` (v2.18.5+, released 2026-04-26/27) restructured its composite action to pass inputs via env-var mapping. Gitea `act_runner` doesn't expand `${{ }}` expressions inside docker-action `env:` blocks reliably, so the literal string `${{ inputs.config }}` reached the binary and broke the 'Check coverage' step.
- Replace the action with a direct `go install` + binary invocation (matching the established Frostmoln pattern).
- Use `--github-action-output` to expose `total-coverage` as a step output, replacing the manual `go tool cover -func | grep | awk` calculations.
- Baseline artifact now stores the percentage directly instead of the full coverage profile.

## Test plan

- [x] `prek run --all-files` passes
- [ ] CI passes on this PR
- [ ] After merge, baseline artifact format propagates on next push to main

Reviewed-on: #150

.gitea/workflows/ci.yaml

@@ -18,11 +18,10 @@ jobs:
         run: go test -race -coverprofile=coverage.txt ./...
 
       - name: Check coverage
-        uses: vladopajic/go-test-coverage@v2
-        with:
-          config: ./.testcoverage.yml
-
-      # Download baseline coverage from main branch (for PRs)
+        id: coverage
+        run: |
+          go install github.com/vladopajic/go-test-coverage/v2@latest
+          go-test-coverage --config ./.testcoverage.yml --github-action-output
       - name: Download baseline coverage
         if: gitea.event_name == 'pull_request'
         uses: actions/download-artifact@v3
@@ -30,14 +29,12 @@ jobs:
           name: coverage-baseline
           path: ./baseline
         continue-on-error: true
-
-      # Compare coverage against baseline (for PRs)
       - name: Compare coverage
         if: gitea.event_name == 'pull_request'
         run: |
-          CURRENT=$(go tool cover -func=coverage.txt | grep "^total:" | awk '{print $NF}' | tr -d '%')
+          CURRENT="${{ steps.coverage.outputs.total-coverage }}"
           if [ -f ./baseline/coverage.txt ]; then
-            BASE=$(go tool cover -func=./baseline/coverage.txt | grep "^total:" | awk '{print $NF}' | tr -d '%')
+            BASE=$(cat ./baseline/coverage.txt)
             echo "Base coverage: ${BASE}%"
             echo "Current coverage: ${CURRENT}%"
             if [ "$(echo "$CURRENT < $BASE" | bc -l)" -eq 1 ]; then
@@ -49,8 +46,9 @@ jobs:
             echo "No baseline coverage found, skipping comparison"
             echo "Current coverage: ${CURRENT}%"
           fi
-
-      # Upload coverage as baseline (only on main)
+      - name: Save coverage baseline
+        if: gitea.ref == 'refs/heads/main'
+        run: echo "${{ steps.coverage.outputs.total-coverage }}" > coverage.txt
       - name: Upload coverage baseline
         if: gitea.ref == 'refs/heads/main'
         uses: actions/upload-artifact@v3
@@ -58,19 +56,17 @@ jobs:
           name: coverage-baseline
           path: coverage.txt
           retention-days: 90
-
-      # Post coverage to PR comment
       - name: Post coverage comment
         if: gitea.event_name == 'pull_request'
         env:
           GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
           GITEA_URL: ${{ gitea.server_url }}
         run: |
-          COVERAGE=$(go tool cover -func=coverage.txt | grep "^total:" | awk '{print $NF}')
+          COVERAGE="${{ steps.coverage.outputs.total-coverage }}"
           curl -X POST "${GITEA_URL}/api/v1/repos/${{ gitea.repository }}/issues/${{ gitea.event.pull_request.number }}/comments" \
             -H "Authorization: token ${GITEA_TOKEN}" \
             -H "Content-Type: application/json" \
-            -d "{\"body\": \"## Coverage Report\n\nTotal coverage: **${COVERAGE}**\"}"
+            -d "{\"body\": \"## Coverage Report\n\nTotal coverage: **${COVERAGE}%**\"}"
 
   vulnerabilities:
     runs-on: ubuntu-latest