v1.3.1 — chore: version sync + auto-dogfood

Chore release bundling three small fixes after the v1.3.0 comet-tail ship:

• Manifest sync: package.json version field moved 1.2.0 → 1.3.1 to match the tag.
• README hero: now reads from comet-graph/comet.svg (self-dogfood, kiaquila account, auto-refreshing) instead of the static showcase/comet.svg snapshot.
• Dogfood auto-trigger: .github/workflows/comet-graph.yml now fires on push: branches: [main] with a paths: filter (src/, dist-action/, action.yml, scripts/fetch-contributions.mjs), so source-repo hero updates on every action-code merge instead of the weekly Monday tick.

No renderer or action-surface changes — v1.3.0 output SVG is byte-identical.

Spec: specs/010-version-sync-dogfood-auto.