[idea] Attach a client to a plain shell pane (pane attach, no agent)

[eddof13]

Is there a supported way to attach a full-screen client to a plain shell pane — one with no agent?

agent attach only accepts panes that have an agent, and there's no pane attach. On a plain shell pane it fails at the resolve step:

$ herdr pane split           # a shell pane, agent = None
$ herdr agent attach w2:p13
{"error":{"code":"agent_not_found","message":"agent target w2:p13 not found"}}

The only workaround I've found is herdr pane report-agent <pane> --agent shell --state idle, which makes the pane attachable — but then it shows up in the agents section and takes over lifecycle authority, so a real agent that later starts in that pane isn't detected.

Would you be open to a first-class pane attach <pane-id> [--takeover] that streams any pane the way the TUI already does, so a shell can be viewed without pretending it's an agent? Happy to open a PR if it's something you'd take.