changelog: extract markdown source to FerrLabs/Changelog repo + propagate to product sites

[BryanFRD]

Today the changelog markdown lives in FerrLabs-Cloud/site/src/content/changelog/ and renders only on ferrlabs.com/changelog/. Each product site (ferrflow.com, ferrtrack.com, ferrvault.com, ferrgrowth.com, agents.ferrlabs.com) should get its own filtered /changelog/ page so:

• SEO: per-product changelog pages match <product> changelog search intent.
• UX: visitor on ferrflow.com sees the FerrFlow changelog without leaving the product site.
• Cross-link: each entry on a product site links back to ferrlabs.com/changelog/ for the cross-product view.

Proposed architecture

New public repo FerrLabs/Changelog with just markdown files at the root (matches the schema in CLAUDE.md). Source of truth for every site.

Each Astro site adds a build step that pulls the .md files at build time:

"prebuild": "degit FerrLabs/Changelog#main src/content/changelog --force"

Then the existing content collection works as-is, just filtered:

const entries = await getCollection('changelog', e => e.data.product === 'ferrflow');

A repository_dispatch from FerrLabs/Changelog to each consumer triggers a rebuild when entries land. Or hourly schedule: cron, simpler.

Migration steps

1. Create FerrLabs/Changelog (public, MIT or no-license).
2. Move the 3 existing .md files from FerrLabs-Cloud/site/src/content/changelog/ to the new repo root.
3. Add the prebuild step + content collection to: FerrFlow-Cloud (Starlight needs care), FerrTrack-Cloud, FerrVault-Cloud, FerrGrowth-Cloud, FerrAgents.
4. Add filter UI on ferrlabs.com/changelog/ (query-string ?product=).
5. Update CLAUDE.md: PRs add their entry to FerrLabs/Changelog (not FerrLabs-Cloud).
6. Cross-link footer of each product changelog page → ferrlabs.com/changelog/.

Why not now

Decision: do it once we have ≥5 entries to avoid setting up the cross-repo machinery for almost nothing. Today we have 3.