Refactor plan render pipeline to use @refrakt-md/html

Acceptance Criteria

• render-pipeline.ts calls renderFullPage() from @refrakt-md/html instead of building HTML strings
• The bespoke HTML shell template is removed
• getLuminaBaseCss() is removed — @refrakt-md/lumina is no longer a dependency
• escapeHtml utility is removed (handled by the HTML adapter)
• Built-in theme CSS is served at /__plan-theme.css (serve) or written to theme.css (build)
• SSE hot reload script is injected via headExtra option
• planLayout from WORK-037 is used for structural placement
• Nav region from WORK-038 is passed as regions.nav
• plan serve produces identical visual output as before
• plan build produces identical static output as before
• --theme flag still works for default, minimal, and custom paths
• --base-url flag still works correctly
• All existing plan serve/build tests pass

Approach

1. Add @refrakt-md/html as a dependency of @refrakt-md/plan
2. Remove @refrakt-md/lumina dependency
3. Assemble a minimal HtmlTheme with planLayout
4. Replace the HTML shell with renderFullPage() calls
5. Move theme CSS from inline <style> to external stylesheet references
6. Update serve command to serve CSS and behaviors at dedicated routes
7. Update build command to write CSS and behaviors as separate files

References

• Plan Site via HTML Adapter (Plan Site via HTML Adapter)
• Define planLayout in @refrakt-md/transform (planLayout definition)
• Build nav region builder for plan site (nav region builder)