feat(vscode): support Agent tool and passthrough SubagentEvent fields

[tempurai]

Summary

• Render the Agent tool using the same UI as Task in the webview
• Fall back to subagent_type when subagent_name is missing on Task/Agent calls
• Treat Agent results like Task results when resolving subagent steps
• Allow unknown fields on SubagentEventSchema via .passthrough() for forward compatibility

Test plan

• Trigger a tool call named Agent and confirm it renders with the Task-style card
• Confirm subagent steps still attach to their parent tool call
• Confirm existing Task flows are unaffected