fix: render existing agents when layout is already loaded

[yshyuk]

Problem

In standalone mode, restored/external agents show up in the debug view but their characters never appear on the office canvas after a page load or refresh.

Cause

On webview connect, the standalone server sends layoutLoaded before existingAgents (server/src/clientMessageHandler.ts). The existingAgents handler in webview-ui/src/hooks/useExtensionMessages.ts unconditionally buffers incoming agents into pendingAgents, which is only flushed by a subsequent layoutLoaded. Since that message has already passed, the buffer never flushes: React state (setAgents) is updated — hence the debug view shows the agents — but os.addAgent() is never called, so no characters are created.

Fix

In the existingAgents handler, if the layout is already ready (layoutReadyRef.current), add the characters immediately via os.addAgent() (deduped against os.characters). The buffering path is kept for the case where existingAgents arrives before layoutLoaded.

How to reproduce / verify

1. npx pixel-agents with previously persisted external agents (or adopt a couple of running Claude Code sessions with Watch All Sessions on).
2. Open/refresh http://localhost:3100.
3. Before: office renders with no characters; debug view lists the agents. After: characters render at their seats immediately.