Add GitHub Action to deploy landing page and docs on main

## Summary

Deploy the public site automatically when changes land on `main`.

`pnpm site:deploy` already builds the combined static bundle (Astro landing at `/` + Blume docs at `/docs/`) and publishes it to the `fabricate.dmach.in` Cloudflare Worker. CI currently checks and releases, but does not deploy the site.

## Ask

Add a GitHub Action that:

- Runs on merge/push to `main`
- Builds with `pnpm site:build` (or `pnpm site:deploy`)
- Deploys via Wrangler to the existing `fabricate-site` Worker / `fabricate.dmach.in` custom domain

Path-filter to `apps/landing`, `apps/docs`, `apps/site`, and related site content if that keeps the workflow cheap.

## Notes

- Local deploy path is documented in `apps/site/README.md`
- Wrangler config lives in `apps/site/wrangler.jsonc`
- Will need Cloudflare credentials as GitHub Actions secrets

Start with apps/site/README.md for the local deployment path and apps/site/wrangler.jsonc for the existing Worker configuration; then inspect the current CI and release workflows. Add a workflow triggered by changes on main, using the documented site build or deploy command and the required GitHub Actions secrets. Done means the landing page and docs deploy to fabricate-site at fabricate.dmach.in.

Written by the indexing model from the issue text.