Add CMS site agent guidance

AGENTS.md

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+# Happy Tails Astro and internal CMS guidance
+
+## Repository role
+
+- This is the active production source for the Happy Tails primary and blog
+  sites. The remote default is protected `main`; the slash-free `cms` branch is
+  the long-lived internal editor branch.
+- Read `README.md`, `docs/content-model.md`, `docs/decap-cms-pilot.md`, and the
+  company-wide `../../docs/operations/optional-decap-cms.md` before changing
+  content or CMS behavior.
+
+## Content and release boundaries
+
+- Keep public content in the typed Markdown collections and assets in the
+  site-owned public paths. Do not fabricate business facts, claims, reviews,
+  availability, or form success.
+- CMS editors may change only the approved content/media paths. Do not expand
+  CMS access to workflows, source, scripts, schemas, manifests, Wrangler files,
+  secrets, or deployment controls.
+- `cms` produces a no-index preview. Production requires validation, human
+  review, and a protected merge to `main`; publishing content never authorizes
+  an automatic production merge.
+- Keep the primary and blog as isolated builds and Workers. A deployment must
+  build and upload only its selected `SITE_KEY` output.
+- DNS, custom domains, contact routing, Gitea OAuth/CORS, branch protection,
+  and production deployment require explicit authorization beyond a source edit.
+
+## Verification
+
+- Run `npm run verify:cms` for CMS/configuration changes.
+- Run both complete site pairs after shared content or CMS machinery changes:
+  `SITE_KEY=www npm run build:site`, `SITE_KEY=www npm run verify:build`,
+  `SITE_KEY=blog npm run build:site`, and
+  `SITE_KEY=blog npm run verify:build`.
+- Preserve image optimization, metadata removal, draft filtering, no-index
+  previews, canonical production metadata, sitemaps, and real 404 behavior.