ci: wire gist-hosted coverage badge

Add schneegans/dynamic-badges-action step to the test workflow, gated on
push-to-main from ubuntu-latest only. Extracts total coverage from
coverage.out and writes it to gist 7f1197011c2f419e7192c2ba4d530195.
README gains a 4th badge pointing at the gist endpoint.

Requires the GIST_TOKEN repo secret (gist scope only).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: clairevnext

.github/workflows/test.yaml

@@ -54,3 +54,28 @@ jobs:
 
       - name: Run benchmarks
         run: go test -bench=. -benchmem ./...
+
+      # Coverage badge: on a successful push to main from ubuntu-latest only,
+      # extract the total coverage percent from coverage.out and write it to
+      # the repo's public gist via schneegans/dynamic-badges-action. shields.io
+      # reads the gist via endpoint= mode. Requires GIST_TOKEN repo secret
+      # (gist scope only). PRs and Windows/macOS runs skip this step.
+      - name: Extract coverage percent
+        id: coverage
+        if: matrix.os == 'ubuntu-latest' && github.event_name == 'push' && github.ref == 'refs/heads/main'
+        run: |
+          pct=$(go tool cover -func=coverage.out | awk '/^total:/ {gsub("%",""); print $3}')
+          echo "pct=$pct" >> "$GITHUB_OUTPUT"
+
+      - name: Update coverage badge gist
+        if: matrix.os == 'ubuntu-latest' && github.event_name == 'push' && github.ref == 'refs/heads/main'
+        uses: schneegans/dynamic-badges-action@v1.7.0
+        with:
+          auth: ${{ secrets.GIST_TOKEN }}
+          gistID: 7f1197011c2f419e7192c2ba4d530195
+          filename: coverage.json
+          label: coverage
+          message: ${{ steps.coverage.outputs.pct }}%
+          valColorRange: ${{ steps.coverage.outputs.pct }}
+          minColorRange: 50
+          maxColorRange: 95

README.md

@@ -3,6 +3,7 @@
 [![Go Reference](https://pkg.go.dev/badge/github.com/bold-minds/each.svg)](https://pkg.go.dev/github.com/bold-minds/each)
 [![Build](https://img.shields.io/github/actions/workflow/status/bold-minds/each/test.yaml?branch=main&label=tests)](https://github.com/bold-minds/each/actions/workflows/test.yaml)
 [![Go Version](https://img.shields.io/github/go-mod/go-version/bold-minds/each)](go.mod)
+[![Coverage](https://img.shields.io/endpoint?url=https://gist.githubusercontent.com/clairevnext/7f1197011c2f419e7192c2ba4d530195/raw/coverage.json)](https://github.com/bold-minds/each/actions/workflows/test.yaml)
 
 **Find, filter, group — slice operations Go stdlib skipped.**