Dependency visualization for plan site

Acceptance Criteria

• During the register pipeline phase, entity content is scanned for ID reference patterns
• A bidirectional relationship index is built in the EntityRegistry
• Each entity page displays a "Relationships" section showing linked entities
• Each reference is a live link with an inline status badge
• Relationships are categorized: "Blocked by", "Blocks", "Related"
• Blocked items show a visual indicator in the sidebar when blockers are unresolved
• Relationships render via a computed layout slot or region (not manual HTML injection)

Approach

Extend the plan package's pipeline hooks. In register(), scan each entity's content for (WORK|SPEC|BUG|ADR)-\d+ patterns and index relationships. The relationships section is surfaced via the shared layout engine — either as a computed slot in planLayout (similar to how computed.toc works) or as an injected region. This avoids manual HTML injection in postProcess() and keeps the rendering declarative.

Once WORK-050 converges the plan renderer with the shared layout engine, the relationships section can sit in the right-side panel alongside the ToC, providing an "on this page" + "related entities" sidebar similar to documentation sites.

References

• Plan Site UX at Scale (Plan Site UX at Scale — Feature 6)
• Add Entity Registration to @refrakt-md/plan (entity registration — the registry this builds on)
• Converge plan site renderer with shared layout engine (renderer convergence — provides the layout slot mechanism)