feat: add user-owned server routes with prepared publication assets

[MohamedH1998]

Summary

Allows user-owned Astro routes to keep Astro’s native rendering behavior while Nimbus validates only the routes it owns. It also makes Markdown and llms.txt agent endpoints safe to render on request.

What changed

• Validates Nimbus-managed routes by route pattern and entrypoint.
• Leaves unrelated user and integration routes under Astro’s control.
• Adds canonical agent endpoint helpers under @cloudflare/nimbus-docs/agent-endpoints.
• Preserves the existing publication and build compatibility entrypoints.
• Moves prepared endpoint content into content-addressed assets instead of server bundles.
• Supports Cloudflare bindings and Node-compatible asset loading.
• Adds asset containment, cleanup, and route-reporting safeguards.
• Includes concrete custom on-demand routes in .nimbus/routes.json.
• Deduplicates the deployment root in sub-path sitemaps while leaving trailing-slash policy to Astro.
• Updates starter routes, documentation, registry guidance, and end-to-end coverage.

[pkg-pr-new]

pnpm add https://pkg.pr.new/@cloudflare/create-nimbus-docs@122

pnpm add https://pkg.pr.new/@cloudflare/nimbus-docs@122

commit: a065d76