Define `planLayout` in `@refrakt-md/transform`

Add a planLayout alongside the existing docsLayout and blogArticleLayout in the transform package's layout configs.

The plan layout is structurally simple: a fixed sidebar slot for navigation and a main content area. No TOC, no breadcrumbs, no version switcher — just the two slots.

Acceptance Criteria

planLayout is exported from packages/transform/src/layouts.ts
Layout has sidebar slot (source: region:nav) and main slot (source: content)
Layout block name is plan
Layout is importable as import { planLayout } from '@refrakt-md/transform'
Existing layout tests still pass

Approach

Follow the pattern of docsLayout and blogArticleLayout in packages/transform/src/layouts.ts. The plan layout is simpler than either — no computed content slots, no behaviors.