[Epic] Let partners onboard and publish plugins without bespoke support

## Problem statement

Partner plugins require bespoke guidance and maintainer intervention to adopt repository conventions, quality gates, ownership, and publication workflows.

## Outcome

Partners can onboard, validate, and publish plugins through a documented self-service process.

## Goals

- Publish contributor and partner onboarding guidance
- Define ownership, support, review, and publication expectations
- Provide templates and validation paths for new plugins
- Make quality and release gates explicit before submission

## Success criteria

- [ ] A partner can scaffold and validate a plugin without bespoke setup
- [ ] Ownership and support boundaries are documented
- [ ] Publication requirements and evidence are clear

## Tracked work

#2795, #2796, #2797, #2798, #2804