Desktop: render the live agent workbench

[qwen-code-dev-bot]

Parent

Parent: #105

What to build

Turn the selected local session into a readable workbench: stream real user and assistant output, tool activity, file changes, and failures into chronological cards. Add a fixed composer that exposes the sanitized model label and effective permission state.

Acceptance criteria

• The default workspace distinguishes user, assistant, tool, diff, and error events.
• Streaming updates are tied to one real session and retain ordering across reconnect.
• The composer displays current model and permission status without exposing secrets.
• A selected diff or tool card opens enough context to inspect the event receipt.
• Tests cover state reduction and one real streamed request through the bridge.
• A PR-bound tmux E2E receipt and screenshot show a real streamed interaction.

Blocked by

• Desktop: connect a real local session and Terminal view #107