[Feature] Treat subagents as first-class observable sessions: attach, monitor, manage individually

What feature would you like to see?

Make subagents first-class observable sessions: while a subagent (or swarm member) runs, the user should be able to attach to it like a session — watch its live event stream, and manage it individually (stop / steer / resume) — instead of it being an opaque one-shot call that only reports back at the end.

Today:

• A background Agent task exposes only its output buffer. (#667 and PR #2130 add a live text transcript to that buffer — but it is still a log you poll, not a session you can attach to.)
• Foreground subagents render progress only as a transient grouped card; once it scrolls away there is no way back to the subagent's own timeline.
• Swarm members (AgentSwarm) are worse: they never register background tasks at all, so there is no per-member output, no per-member status, and no per-member stop — see #1957 ("Swarm无法查看进度").
• Management is all-or-nothing: interrupting cancels every sibling; you cannot stop or steer one subagent.

Related existing work: #667 (live task output), #1957 (swarm progress), PR #1139 (/agents status command), PR #1210 (session debugging timeline), PR #1645 (stop individual swarm members from web UI). None of them offers attach-to-session semantics; this request is for the missing primitive underneath all of them.

Implementation sketch (from reading the source)

The pieces mostly exist; what is missing is exposing them per-subagent:

1. Event source — every subagent already emits AgentEvents (turn.started, thinking.delta, tool.call.started, …). In-process subscription exists (Agent.onEvent, added by the #667 fix); on the wire, kap-server's sessionEventBroadcaster already fans out session events, and every subagent has a stable agentId (session metadata, subagent.spawned events). Add an agent-scoped subscription (transcript subscribe_v2 recently landed for sessions — the same channel filtered by agentId gives a live subagent transcript for free).
2. Roster — the live subagent roster is already carried in session snapshots (#1623) and the web agents dock (#1754). Promote it to a first-class list in the TUI (the direction PR #1139 takes with /agents) where each entry opens the attach view.
3. Attach view — render the agent's AgentEvent stream with the same components the TUI already uses for the main agent (the redesigned subagent card, #1345, and the vis timeline, #1210, are prior art). Attach/detach is a pure view operation; the subagent keeps running either way.
4. Management — steer = Agent(resume=agentId, ...) (exists today); stop = the same abort path TaskStop uses for agent tasks, extended to swarm members by wiring each SubagentBatch attempt controller to a member id (PR #1645 does this for the web UI).

Suggested first increment: (1) + (2) read-only — a /agents panel where selecting a running subagent shows its live transcript. That alone closes #1957's "did it hang?" problem and needs no new control surface.

Additional information

Filed after a source read of packages/agent-core (v1) and kap-server — I'm not a regular contributor, so the sketch above may miss a simpler seam (for example, reusing the background-task output streaming from #667 instead of a new subscription channel). Very happy to adjust the proposal around whatever shape maintainers prefer.

Start by reading packages/agent-core and kap-server, especially Agent.onEvent, sessionEventBroadcaster, transcript subscribe_v2, and the session snapshots used by /agents. Trace how agentId and SubagentBatch attempts are represented; the first increment is done when selecting a running subagent in /agents opens a live, read-only transcript without affecting its execution.