Converge plan site renderer with shared layout engine

The plan site's renderPage() in runes/plan/src/commands/render-pipeline.ts manually assembles HTML using a local planTheme/planLayout definition, bypassing the shared layout engine in @refrakt-md/transform. Meanwhile, packages/transform/src/layouts.ts already exports a planLayout config (WORK-037) that goes through the standard layout engine but isn't used by the plan site's renderer.

Converge the plan site's renderer to use the shared layout engine so it can leverage declarative layout features — computed ToC, conditional slots, behaviors array, computed content — rather than hand-building HTML. This unblocks ToC ("on this page"), dependency visualization (WORK-049), keyboard nav behaviors (WORK-048), and any future layout-level features without custom rendering code.

Acceptance Criteria

planLayout in packages/transform/src/layouts.ts is extended with computed.toc (headings, minLevel 2, maxLevel 3, minCount 2)
planLayout main slot wraps content with a ToC aside, matching the docs layout pattern
Plan site's renderPage() delegates to the shared layout engine instead of manually assembling HTML
The sidebar nav region (status groups, search input, dashboard link) renders identically to current output
Sidebar behavior scripts (collapse, search/filter) are injected via the layout engine or a behavior entry point
Copy-to-clipboard behavior is injected via the layout engine or a behavior entry point
"On this page" ToC appears on entity pages with 2+ headings
ToC is styled consistently with the docs layout ToC (sticky, right-side aside)
Plan-specific CSS for the ToC aside (.rf-plan-toc or reusing .rf-docs-toc pattern) is added
All existing plan site tests pass
refrakt plan serve and refrakt plan build produce equivalent output (modulo new ToC)

Approach

Phase 1 — Extend planLayout config

Add computed.toc and restructure the main slot in packages/transform/src/layouts.ts to include a ToC aside and content wrapper, following the docsLayout pattern:

computed: {
    toc: {
        type: 'toc',
        source: 'headings',
        options: { minLevel: 2, maxLevel: 3 },
        visibility: { minCount: 2 },
    },
},

Add a content wrapper with conditionalModifier for has-toc and a ToC aside sourced from computed:toc.

Phase 2 — Wire renderPage to the layout engine

Replace the manual HTML assembly in renderPage() with a call through the shared layout engine. The nav region builder output feeds into the region:nav slot. Page content feeds into the content source. The layout engine handles structure, conditional slots, and computed content.

The local planTheme manifest and planLayout in render-pipeline.ts can be removed once the shared config is authoritative.

Phase 3 — Migrate inline behaviors

Move the inline <script> blocks (sidebar collapse/search, copy-to-clipboard) to proper behavior entry points that the layout engine can inject. Options:

Add them to planLayout.behaviors array if they can be standard @refrakt-md/behaviors modules
Or keep them as plan-specific scripts injected via the engine's script slot

Phase 4 — Add ToC CSS

Add plan-specific ToC styles (.rf-plan-toc) to runes/plan/styles/default.css, following the sticky aside pattern from packages/lumina/styles/layouts/docs.css. The ToC should sit to the right of .rf-plan-main content.

Non-goals

Migrating the dashboard page to a different rendering approach (it can continue using generateDashboardContent())
Adding breadcrumbs to the plan layout (not needed for flat entity pages)
Adding a header bar or top navigation (plan site stays minimal)

References

WORK-037 (original planLayout definition — this extends it)
WORK-039 (HTML adapter refactor — prerequisite infrastructure)
WORK-049 (dependency viz — unblocked by this, can use a computed slot)
WORK-048 (keyboard nav — can use the behaviors array)
SPEC-014 (Plan Site via HTML Adapter)
SPEC-015 (Plan Site UX at Scale)