[liaowang11]

Two changes, plus a review round on top of them.

Where a side conversation opens

New agent-shell-side-display-action, defaulting to a 40% window on the
right. The parent keeps using upstream's agent-shell-display-action; it
is an ordinary shell. Closing a side conversation deletes its side window
rather than handing the parent a narrow strip. Toggling with both on
screen now selects the other window instead of duplicating the side.

Setting the option to display-buffer-same-window reproduces the old
behaviour exactly, so nothing needed a special case. Under viewport
interaction the action is not consulted: the viewport owns its layout.

One handback path

Findings always go into the parent's compose buffer now, whatever
agent-shell-prefer-viewport-interaction says. That setting decides
where the user's own prompts go and has no business deciding what happens
to findings.

This deletes the wait-for-idle machinery: --stage-when-idle,
--parent-busy-p, the pending flag, its mode-line marker, and the
agent-shell-side-handback-submit option. A compose buffer takes text
mid-turn and offers send, queue, or steer from its own keys. Waiting
could only ever offer send, and only later.

Review fixes (624b7e0)

• conclude signalled "Not yet supported" when run from a compose
  buffer. agent-shell-insert dispatches on the current buffer, not the
  target. Both call sites now address the side shell directly.
• Appending findings reset the compose disposition, silently turning a
  queued draft into a steer. The in-progress draft's disposition is now
  carried through.
• The new side-window branch in --close was dead: kill-buffer already
  deletes a dedicated side window. Removed.
• Live probes and docs updated; the compose buffer being selected on
  arrival is now stated in the README.

Verification

make check green, 107/107. The three new tests fail against 30236f8,
as does an existing viewport test that had been passing vacuously
because the stub did not dispatch. Byte-compiles clean against the real
agent-shell, shell-maker and acp; checkdoc clean.

Not verified: make live-check has not been re-run against a real agent
since the handback moved. Its probes were updated to match but the run
has not happened, and DESIGN.md and the README now say so.