fix(ui): track RPC-spawned agent activity

[xz-dev]

Summary

• create the normal live activity tracker for agents spawned through the cross-extension RPC used by TaskExecute
• register RPC-spawned activity with the agent widget and FleetView
• add an extension-level regression test covering a running tool being shown instead of a permanent thinking… fallback

Problem

TaskExecute starts agents through subagents:rpc:spawn, which called AgentManager.spawn() directly. Unlike the Agent tool path, that path never created an AgentActivity tracker or supplied its callbacks.

This produced contradictory UI: the header's tool-use count continued increasing and Conversation Viewer showed real work, while the activity line stayed at thinking… for the entire run.

Fix

The RPC handler now receives a narrow manager wrapper that creates the existing tracker before spawning, composes its callbacks into the spawn options, and registers the returned agent ID with the existing widget/FleetView activity map. Direct Agent spawns and scheduled agents remain unchanged.

Verification

• regression test proved RED before the fix (thinking… while a bash tool was active)
• npm run lint
• npm run typecheck
• npm run test — 756 passed, 5 skipped
• npm run build
• git diff --check