[SDK-10] Plugin-author documentation page

Author the plugin-author docs (walkthrough + API ref).

## Status

**Blocked on #41 (docs site migration)** — once the docs site lands, write the page there. Until then, draft in a holding folder at `docs/_drafts/plugin-author-guide.md` so the prose is ready to drop in.

## Scope

- Walkthrough: write a 'hello-world' plugin against the SDK, register one middleware, run it against a sample build.
- Reference: enumerate all six extension points (SDK-6a..6f), each with a code sample.
- Conflict resolution behaviour (SDK-4).
- Versioning policy explainer (SDK-5).
- Cross-link to the sample plugin (SDK-8) and testing helpers (SDK-9).

## Depends on

#41 (unblocks the publish), SDK-1..SDK-9 (subject matter).