feat(node-summary): visual shell for node summary sheet

[SobshDev]

Summary

• New src/modules/node-summary/ module exposing NodeSummarySheet — a right-side Sheet with header (name/type/file:line), facts row, signature, overview prose, and Used-by / Uses lists.
• Single-click on a code node opens the sheet (header populated from the real node; body still mock data — AI fetch comes later). Double-click still opens the code view; a short grace window cancels the pending summary when a double-click follows.
• No new dependencies; reuses Sheet, Button, Badge, and Lucide icons.

Test plan

• bun run dev → click any code node, summary sheet slides in from the right with that node's header info
• Double-click a code node → only the code view opens (no summary sheet flash)
• Close the sheet via X or outside click → stays closed until another node is clicked
• bun run tidy passes