iOS UX: add channel-scoped agent profiles with threads and recent tool activity #5067

Summary

Add an agent-specific profile experience on iOS that can be scoped to the current channel and used to browse where the agent is active: channel/thread appearances and recent tool calls.

The existing profile sheet treats agents like general users. From mobile, a user can inspect identity and start a DM, but cannot answer: "Where is this agent being used in this channel?" or "What did it do recently?"

Proposed UX

• Open an agent profile from a member row, mention, typing indicator, or activity view.
• Default to the current channel when launched from a channel or thread.
• Provide swipeable/tabs views for Profile, Threads, and Recent activity.
• In Threads, list the current channel's threads/conversations where the agent was mentioned or participated, with navigation back to each source.
• In Recent activity, show the agent's recent channel-scoped observer/tool transcript with timestamps and source links.
• Allow changing the channel scope when the agent is present in multiple channels.
• Keep owner-only/decrypted observer data subject to the existing authorization model; do not expose private tool payloads to unauthorized members.

Source references

• user_profile_sheet.dart accepts only a pubkey and has no channel/agent activity context.
• user_profile_sheet.dart builds a general identity/profile action sheet rather than an agent workspace view.
• agent_activity_sheet.dart already has a channel-and-agent-scoped activity surface that can be reused.
• observer_subscription.dart defines the existing channel-scoped observer key and transcript state.

Related work

This is narrower than the workspace-wide fleet view in #4169 and the remote-agent roster in #3996. It focuses on an individual agent's channel context and recent work on iOS. See also #3907 for mobile activity visibility and #5062 for the prior THREAD/client UX report.