Show the delegated task in the parent transcript on spawn

[monneyboi]

Problem

When spawn_agent is called, the task is written to a task file and handed to the child via @<taskPath>, but the parent window never shows what was sent. The tool result only reports the subagent id and location:

Spawned subagent in . ...

To see what a subagent is working on, the user has to switch to its tmux window. The parent transcript — the one place that records the whole delegation lifecycle (spawn → report → revise → close) — is missing the most important part: the task itself.

Expected behavior

After a successful spawn, the parent transcript should display the task that was delegated, alongside the id/location info. It should be visible without switching windows, and consistent with how subagent reports are already rendered (fork-result with renderForkMessage, including the collapsed preview for long text).

Fix

In spawn_agent's execute in index.ts, after the spawn succeeds, deliver the task via pi.sendMessage with a new custom type (e.g. fork-spawn), display: true, and no triggerTurn — this is a display-only record, not a new conversation event:

pi.sendMessage(
  {
    customType: SPAWN_TYPE,
    content: `Spawned subagent ${id} in ${location}:\n\n${task}`,
    display: true,
  },
  { triggerTurn: false },
);

Register the type with the existing renderForkMessage renderer so long tasks collapse to the PREVIEW_LINES preview like reports do.