Spawn extra task agents through the backend-neutral pane seam

[h0x91b]

The "+ Agent" action was the third launch path still splitting dev3-<taskId> unconditionally: spawnAgentInTask called tmux.splitWindow directly, so on a native-backed task there was no such session and the launch failed the same way Find bugs (#1228) and AI Review (#1229) did.

It now goes through splitTaskPane — the bare primitive, not an auxiliary purpose, because "+ Agent" deliberately allows several coexisting agent panes while a purpose owns at most one.

• Native: a real coordinator pane, no tmux call and no probe of the default tmux socket.
• Native: no sessionState.panes entry, which is the tmux pane registry handlePaneExited reconciles against live tmux panes — a native pane id there would be a permanent phantom. The agentId/configId write is unchanged on both backends.
• tmux: the same split-window -h with the same -c/-t/-e and the same pane-registry append; the only delta is the seam's DEV3_TASK_SEQ env var, which every other extra pane already carries.
• A dead terminal now reports "the task terminal is not running, so there is no pane to split — start the task first." instead of a tmux socket error, and a failed split leaves no pane and no task write.

Verified live on a native task (extra pane opens beside the agent, both halves equal, second Claude process running, zero tmux sessions for the task, no phantom pane record) plus new regression cases through the real RPC entry point.