ci(coverage): drop --depth=1 on the base fetch — fix diff-cover 'no merge base' (#49)

The patch-coverage gate fetched the base with --depth=1, leaving no merge-base
with a PR branch behind base → diff-cover crashed with 'no merge base'. Checkout
is fetch-depth:0, so a plain fetch resolves it.

Co-authored-by: Claude <noreply@anthropic.com>

Author: mastermanas805

.github/workflows/coverage.yml

@@ -61,7 +61,9 @@ jobs:
         if: github.event_name == 'pull_request'
         working-directory: common
         run: |
-          git fetch origin "${{ github.base_ref }}" --depth=1 || true
+          # Full base history (NOT --depth=1): a shallow base fetch shares no merge-base
+          # with a PR branch that is behind base → diff-cover "no merge base" crash.
+          git fetch origin "${{ github.base_ref }}" || true
           diff-cover coverage.xml \
             --compare-branch="origin/${{ github.base_ref }}" \
             --fail-under=100