[FEATURE] Supported compaction instructions, and a kick-off prompt after compaction

1. Compaction instructions as a setting. Automatic and idle compactions use the default template. On 2.1.296 a successful PreCompact hook's stdout is appended to the instructions, which works but is undocumented. Please document that or add a compactInstructions setting, so a user can say what must survive: standing instructions with their scope, settled decisions with reasons, what to drop. In long sessions the default summary can widen a scoped go into a standing one.
2. A kick-off prompt, prefilled. After compaction, let the summary (or a hook) propose the next user message and place it in the input box, unsent, so the user can edit and send it. initialUserMessage sends without review, and only at session start.

With a manual /compact the user can read the instructions before it runs. An automatic or idle compaction gives no such chance, so a prefilled kick-off afterwards would be the only point where they can review what carried over. It is also short, so easier to read than a long summary, and a wrong scope or a decision that was never made can be caught before the session acts on it. The closest workaround today is a PostCompact hook that shows part of the summary as a notice, to copy by hand.

Related: #14160, #98747, #101070.