Open real auxiliary panes on the native terminal backend

[h0x91b]

On a task using the native terminal backend, starting the dev server or running
Rebase never produced a visible pane. Both actions split into dev3-<id>, a tmux
session a native task does not have. The two halves failed differently, which is
why one bug looked invisible and the other looked broken:

• Dev Server started the dev script in a tmux session dev3-dev-<id>, then
  failed the viewer split inside a best-effort catch. The server ran with ports
  bound and no pane anywhere — including a tmux session a native task should never
  touch at all.
• Rebase — and Merge / Push / Open PR, which share one helper — threw
  tmux split-window failed instead of doing anything.

The seam

src/bun/task-aux-panes.ts owns every pane an action opens. It picks tmux split vs
native SplitTree pane, replaces the pane a purpose already owns instead of stacking
a second one, and hands focus back to the pane that had it — the native coordinator
makes a new pane active on split, so the agent would otherwise lose input.

Ownership is derived, never stored: a pane is re-found by matching the command it
was launched with, exactly as the tmux code already did with #{pane_start_command}.
Nothing is cached in RAM (which a restart would lose while the pane lives on) and
nothing new is written under ~/.dev3.0/. Pane labels fall out of the same lookup,
so the narrow pager and the close-pane picker can say "Dev Server" instead of "Pane 2".

Callers supply what each backend runs, because they differ: the tmux dev-server pane
runs a re-attach loop into a nested session, while the native pane runs the dev script
itself. tmux behaviour is unchanged and pinned by tests asserting the split arguments.

Two leaks the same blind spot caused

• lifecycle/executor.ts killDevServer explicitly skipped native tasks, with a
  comment claiming a native task owns no dev server. It does — the whole dev-server
  process tree and its pool ports leaked on teardown.
• exitCopyModeAllPanes probed tmux on native tasks.

Behaviour

An action that genuinely cannot run fails honestly: starting the dev server with no
live terminal reports "the task terminal is not running, so it has no pane to split"
and creates no tmux session and no hidden process. That is precisely the case that
used to start an invisible one.