Feature request(prompt management) reusable prompt partials #100897

Is your feature request related to a problem?

When using prompt management, there is no way to share common content across multiple prompts. Common content, such as guardrails, currently has to be declared in every single prompt. It would be good to define that content once and reuse it, instead of repeating it in each prompt.

Describe the solution you'd like

A way to define common content once and reuse it across multiple prompts, so it does not have to be repeated in each one. Some prompt tools call this "prompt partials".

Describe alternatives you've considered

Repeating the shared content in each prompt by hand, which is what has to be done today.

Additional context

• Related prompt management requests (not duplicates): Feature request: prompt management - organize prompts into folders #65578 (organize prompts into folders), Feature request: prompt management - fetch all prompts at a label in one request #95460 (fetch all prompts at a label in one request).
• Docs: https://posthog.com/docs/prompt-management

- [ ] PostHog Cloud, region and project ID: [please provide, you can find both at https://app.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]