ci(coverage): drop --depth=1 on the base fetch — fix diff-cover 'no merge base' (#99)

The patch-coverage gate did 'git fetch origin <base> --depth=1', truncating the
base to one commit with no common ancestor with the PR branch whenever the branch
was behind base — so diff-cover crashed with
'fatal: origin/<base>...HEAD: no merge base' and reded coverage on PRs that were
merely stale (hit repeatedly). The checkout is already fetch-depth:0, so a plain
'git fetch origin <base>' resolves the merge-base reliably.

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

Author: mastermanas805

.github/workflows/coverage.yml

@@ -183,7 +183,9 @@ jobs:
         if: github.event_name == 'pull_request'
         working-directory: worker
         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