[idea] OpenCode: show each session as a separate agent in the Agents panel

[azivkovi]

I run one OpenCode process in a single tab inside a space, and that process has
multiple sessions working at the same time (a turn keeps running in the background
after I switch to another session). In the sidebar's Agents panel, the whole
process shows up as exactly one agent, tracking only the session currently focused
in the TUI.

Today the workaround is one tab per OpenCode process: open a new tab in the space,
launch another opencode there, and I get a second agent row. That works, but it
splits my terminal layout when a single OpenCode process already multiplexes the
sessions fine.

What I'd like: the Agents panel lists every active OpenCode session of that
process as its own agent row — its own state (idle / working / blocked), label,
and session ref for restore — so I can see at a glance which background session
finished or is waiting on a permission prompt without switching sessions in
OpenCode first.

From the docs I understand the current model is "agent = recognized process in a
pane", and the opencode plugin reports a single lifecycle state + session id per
pane, so this would mean letting one pane host multiple agent rows. Related
threads asking for visibility into logical sub-agents inside one pane:
#1274 (Codex subagents), #935 (Pi async subagent lifecycle) — this seems to be a
recurring pattern across agents, and OpenCode's multi-session model is maybe the
cleanest case for it.

If full per-session rows are out of scope, a lighter version — a session count or
a per-session state picker on the pane's agent row — would already help.