[codex] Release ActivityPub bundle contract

[mashbean]

Summary

• Adds ActivityPub Article bundle output for the static publisher.
• Adds activitypub-manifest.json as the handoff contract for the gateway runtime.
• Bumps package metadata to 0.1.9 and publishes @matters/ipns-site-generator@0.1.9 to npm.

Why

matters-server G2-A preflight needs a stable generator package that can emit public long-form Article seed bundles for gateway-core ingestion.

Validation

• Node 18 npm test -- --runInBand passed: 3 suites, 9 tests.
• Node 18 npm run lint passed.
• git diff --check passed.
• npm registry resolves @matters/ipns-site-generator@0.1.9.

Safety

• The package has been published, but this PR remains draft pending maintainer review/merge decision.
• No downstream production deployment is included in this PR.