chore(site): seed announcements feed with welcome banner

[frenchie4111]

Summary

• Adds site/public/announcements.json so harness.mikelyons.org/announcements.json returns a valid feed once this lands.
• Seeds the smallest possible payload — one welcome entry pointing at the GitHub repo — to verify the in-app banner from PR feat(announcements): remote-fetched announcements banner #78 renders end-to-end.
• Schema matches what the renderer expects: { announcements: [{ id, title, href, publishedAt }] }.

Test plan

• Merge, wait for the site deploy, then curl https://harness.mikelyons.org/announcements.json and confirm the JSON is served.
• In a Harness build with PR feat(announcements): remote-fetched announcements banner #78, confirm the banner appears with the welcome title and that clicking it opens the GitHub repo.
• Dismiss the banner and confirm it stays dismissed across restarts.

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