Launch bug hunters as native panes instead of a raw tmux split

[h0x91b]

Summary

Find bugs → Launch hunters failed on a native-backed task with Failed to split pane: error connecting to /private/tmp/tmux-501/ (Socket operation on non-socket).

Root cause: spawnBugHuntersInTask called tmux.splitWindow unconditionally against dev3-task-<id> on pty.getSessionSocket(taskId). A native task has no tmux session, and getSessionSocket falls back to DEFAULT_TMUX_SOCKET — so the launch probed the default tmux socket directory and died. A leftover tmux-only split, same family as the aux-pane fix in seq 1376.

What changed

• src/bun/task-aux-panes.ts — extracted the backend-neutral primitive splitTaskPane() (plus closeTaskPane()) out of openAuxPane(). It takes an explicit tmuxTarget / nativeAnchor / restoreFocus, so callers that split off a pane (not the session) work on both backends. openAuxPane() is now closeAuxPane + splitTaskPane; dev-server and git panes are unchanged.
• src/bun/rpc-handlers/tmux-pty.ts — bug hunters go through splitTaskPane(). On native: real coordinator panes, the prompt delivered via sendPromptToNativePane() (which resolves the pane's writer lease) instead of tmux send-keys, focus handed back to the agent's pane, and no sessionState.panes write (that registry is the tmux one — a native entry would be a permanent phantom). A native launch that cannot open every hunter closes the panes it already opened and throws, instead of silently half-starting.
• tmux tasks keep the identical split geometry, sizes, sessionState bookkeeping and two-step send-keys delivery; a mid-way tmux split failure still continues with the remaining hunters.

Verification

• New backend regressions through handlers.spawnBugHuntersInTask on a native task: N panes opened with the right anchors, zero process spawns (so no tmux call and no default-socket probe), prompt delivered natively, rollback on a failed later split, honest error when the terminal is not running, no sessionState write.
• New renderer regressions entering through the actual Find bugs dialog: default count launches with the picker's defaults, and a backend failure is shown instead of closing the dialog.
• Existing tmux bug-hunter suite passes untouched.
• Live native UI QA: scratch native task → Find bugs → Launch hunters (default 3) → pane-2/3/4 stacked beside the agent's pane-1, all three got the [bug-hunt] prompt, focus stayed on pane-1, no console errors, and no tmux session on either socket.