Add dynamic coverage badge from main branch (#19)

Update CI to extract coverage % on main pushes and store an
\�ndpoint.json\ on a \coverage-data\ branch (same pattern as
[Presidio](https://github.com/microsoft/presidio)). Update README badge
to use shields.io endpoint pointing to that branch.

### Changes
- **ci.yml**: Added 'Update coverage badge' step that runs on push to
main, extracts line coverage from \coverage-summary.json\, generates a
shields.io-compatible \�ndpoint.json\ with color coding, and pushes it
to the \coverage-data\ branch
- **README.md**: Updated coverage badge to use \shields.io/endpoint\
reading from the \coverage-data\ branch

### Notes
- No external services or secrets needed — uses the built-in
\GITHUB_TOKEN\
- \contents: write\ permission scoped only to the coverage job
- Badge color ranges: red (<45%) → yellow → yellowgreen → green →
brightgreen (≥90%)

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dorlugasigal

.github/workflows/ci.yml

@@ -56,11 +56,16 @@ jobs:
           elif (( $(echo "$COVERAGE >= 60" | bc -l) )); then COLOR="yellowgreen"
           elif (( $(echo "$COVERAGE >= 45" | bc -l) )); then COLOR="yellow"
           else COLOR="red"; fi
-          echo "{\"schemaVersion\":1,\"label\":\"coverage\",\"message\":\"${COVERAGE}%\",\"color\":\"${COLOR}\"}" > endpoint.json
+          echo "{\"schemaVersion\":1,\"label\":\"coverage\",\"message\":\"${COVERAGE}%\",\"color\":\"${COLOR}\"}" > /tmp/endpoint.json
           git config user.name "github-actions[bot]"
           git config user.email "github-actions[bot]@users.noreply.github.com"
-          git fetch origin coverage-data:coverage-data 2>/dev/null || git checkout --orphan coverage-data
-          git checkout coverage-data
+          if git fetch origin coverage-data 2>/dev/null; then
+            git checkout coverage-data
+          else
+            git checkout --orphan coverage-data
+            git rm -rf . 2>/dev/null || true
+          fi
+          cp /tmp/endpoint.json endpoint.json
           git add endpoint.json
           git commit -m "Update coverage badge to ${COVERAGE}%" || true
           git push origin coverage-data