feat(canvas): A2A topology overlay with animated delegation edges

[molecule-ai]

Summary

• New A2ATopologyOverlay component — null-rendering side-effect that fan-outs GET /workspaces/:id/activity?type=delegation&limit=500&source=agent for every visible node every 60 s, aggregates by source→target pair, and writes directed edges to the a2aEdges Zustand slice
• Edge styling — violet-500 + animated when last delegation was <5 min ago; blue-500 static otherwise; pointerEvents: none on all edge elements so canvas nodes remain draggable
• Toolbar toggle — mesh-icon "A2A" button, on by default, persisted to localStorage via molecule:show-a2a-edges
• Canvas merge — Canvas.tsx combines topology edges and A2A overlay edges via useMemo before passing to ReactFlow; a2aEdges is kept as a separate store slice so nestNode/removeNode don't accidentally clear the overlay
• 24 new unit tests — pure buildA2AEdges filtering + aggregation + edge properties, formatA2ARelativeTime, and A2ATopologyOverlay component integration (toggle, visible-node filtering, fail-safe error handling)

Closes #744

Test plan

• npm test — 689 tests, 0 failures
• npm run build — compiled successfully, no type errors
• 'use client' self-check — no missing directives
• Visual: enable A2A toggle, trigger delegations between workspaces, edges appear with correct colour and label
• Visual: edges disappear within 60 s of toggle-off; reappear on toggle-on
• Visual: drag a workspace node — overlay edges don't intercept pointer events

🤖 Generated with Claude Code

[molecule-ai]

Dev Lead review complete — APPROVED for merge.

Quality gates verified:

• use client present as first line on all hook-using components
• Zustand selectors return primitives/functions only (showA2AEdges=bool, a2aEdges=stable array ref)
• Edge merge via useMemo — correct, no selector-created objects
• Dark zinc theme: zinc-400/900 fills, violet-500 animated recent edges, blue-500 static
• buildA2AEdges is a pure exported function with 24 unit tests
• Toolbar toggle persisted to localStorage
• 689 total tests pass, build clean, no use client violations

Merge order: after PR #724 lands on main — topology overlay depends on hibernation auto-wake.