Add plan context map sidebar

[woertedetiankong]

Summary

• add a Context sidebar tab for markdown plans with detected code-file mentions
• group mentions into a compact folder-style tree with mention counts and lightweight status badges
• let file rows jump back to the first plan block that mentions that file

Details

• reuses the existing shared code-path extraction rules and adds a repeated-mention helper
• infers Modified, Created, and Deleted only from explicit nearby file-change headings, with conflicts falling back to Mentioned
• reuses /api/doc/exists validation via the existing validated-code-path hook so unresolved paths render muted
• hides the Context tab in archive, annotate, linked-doc, HTML, and active plan-diff views

Validation

• bun test packages/shared/extract-code-paths.test.ts
• bun test packages/ui/utils/planContext.test.ts
• bun run typecheck
• bun run build:hook

[backnotprop]

yo this is sick

[backnotprop]

I'm only concerned about consistency. reviewing some things.