Fix: SSE events not visualized in standalone mode

[matiasdiaz21]

Problem

Agent Flow's visualizer fails to display real-time agent events when running
in standalone mode (pnpm run dev). Events are transmitted from the relay
server to the browser via Server-Sent Events (SSE), but are never visualized
on the canvas or processed by the simulation engine.

Symptoms

• Browser console shows: [SSE] Connected to relay ✓
• Browser console shows: [SSE] Received message: agent-event ✓
• Browser displays: WAITING FOR AGENT SESSION (no agents rendered)
• Events are buffered in sessionEventsRef but never delivered to simulation

Root Causes

1. Unreliable PostMessage Pattern: Original code in use-vscode-bridge.ts
   attempts to route SSE messages through window.postMessage(), which doesn't
   reliably trigger message handlers in same-origin contexts. The VSCodeBridge's
   message handler is designed for cross-origin postMessage (VS Code extension ↔
   webview), not self-messaging.

2. Missing Session Auto-Selection: When SSE events arrive before a session
   is selected (selectedSessionIdRef.current === null), they are routed to
   "background activity" instead of pendingEventsRef, preventing the simulation
   engine from ever receiving them.

Expected Behavior

• SSE events should be processed directly in the hook
• Session should auto-select on first event
• Canvas should display agent and update in real-time
• Timeline should populate with events

Steps to Reproduce

1. Start relay: pnpm run dev
2. Run Claude Code session: claude code "list files"
3. Observe: Browser shows "WAITING FOR AGENT SESSION" instead of visualization

Impact

Standalone mode is completely non-functional without this fix.

For detailed technical analysis, see the related pull request and
FEAT_SSE_EVENT_PROCESSING.md documentation.