feat: MCP get_current_selection tool

[trmquang93]

Summary

Adds a new MCP tool, get_current_selection, that returns the element(s) the user currently has selected in the running Drawd browser app — so an AI agent can act on "this screen" or "this hotspot" without the user pasting IDs.

Selection is 100% runtime state and never persisted, so the tool relies on a tiny localhost HTTP bridge between the app and the MCP server.

How it works

• MCP side — mcp-server/src/selection-bridge.js opens an HTTP listener on localhost:3337 (override with DRAWD_SELECTION_PORT). It stores the latest selection snapshot in memory. If the port is busy it warns on stderr and the stdio server keeps running.
• Tool — mcp-server/src/tools/selection-tools.js reads the snapshot, enriches each item from FlowState:
  • screen → name, description, status, hotspotCount
  • sticky → content, color, x, y
  • connection → fromScreenId, toScreenId, label, action
  • hotspot → screenId, label, action, targetScreenId
  • screenGroup → name, screenIds
  • comment → text, targetType, targetId, resolved
  • Stale (>60s) snapshots return { selection: null, reason: \"no_recent_selection\", hint }.
• App side — src/hooks/useSelectionReporter.js watches every selection slot (canvasSelection, selectedScreen/Sticky/ScreenGroup/Connection/CommentId, selectedHotspots, and hotspotInteraction for single-hotspot select), de-dups, debounces 150 ms, and POSTs to the bridge. Fetch errors are swallowed so the app is unaffected when the bridge isn't running.

Test plan

• npm run lint passes
• npm test — 648 tests pass
• mcp-server/npm run build succeeds; node dist/index.js boots with Selection bridge listening on http://localhost:3337
• POST/GET round-trip verified via curl
• Tool enrichment verified for screen, sticky, connection, hotspot, screenGroup, comment
• EADDRINUSE path warns on stderr without crashing
• Live end-to-end verified in the browser for screen, hotspot, connection, sticky selections
• Reviewer: run npm run dev + MCP server, select elements, call get_current_selection from an MCP client