ci: refresh README star history chart twice daily

Point the README chart at a committed assets/star-history.svg instead of
the star-history API URL so GitHub serves it directly and bypasses the Camo
proxy cache. A scheduled workflow regenerates the SVG at 06:00/18:00 UTC,
busting star-history's server-side cache, and commits it when it changes.

.github/workflows/star-history.yml

@@ -0,0 +1,61 @@
+name: Refresh star history chart
+
+on:
+  schedule:
+    # Twice daily at 06:00 and 18:00 UTC.
+    - cron: "0 6,18 * * *"
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+env:
+  DO_NOT_TRACK: "1"
+  KTX_TELEMETRY_DISABLED: "1"
+  NEXT_TELEMETRY_DISABLED: "1"
+
+concurrency:
+  group: star-history-refresh
+  cancel-in-progress: true
+
+jobs:
+  refresh:
+    name: Regenerate assets/star-history.svg
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Fetch fresh star-history SVG
+        run: |
+          set -euo pipefail
+          # cachebust forces star-history to regenerate instead of serving its
+          # own server-side cache; --location follows the slug-normalizing 301.
+          url="https://api.star-history.com/svg?repos=Kaelio/ktx&type=Date&cachebust=${GITHUB_RUN_ID}-${GITHUB_RUN_ATTEMPT}"
+          curl --fail --location --silent --show-error \
+            --retry 3 --retry-delay 5 --max-time 60 \
+            -o assets/star-history.svg.new "$url"
+          # Guard against error pages / truncated responses before overwriting.
+          if ! grep -q "</svg>" assets/star-history.svg.new; then
+            echo "Downloaded file is not a valid SVG; aborting." >&2
+            exit 1
+          fi
+          if [ "$(wc -c < assets/star-history.svg.new)" -lt 1000 ]; then
+            echo "Downloaded SVG is suspiciously small; aborting." >&2
+            exit 1
+          fi
+          mv assets/star-history.svg.new assets/star-history.svg
+
+      - name: Commit if changed
+        run: |
+          set -euo pipefail
+          if git diff --quiet -- assets/star-history.svg; then
+            echo "Star-history chart unchanged; nothing to commit."
+            exit 0
+          fi
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git add assets/star-history.svg
+          # [skip ci] keeps this housekeeping commit from triggering KTX CI.
+          git commit -m "chore: refresh star history chart [skip ci]"
+          git push