feat(tui): grouped prompt undo that survives submits and keeps pasted text

[Nowaker]

Note: I'm aware v1 is now the maintenance branch and new features go to v2 only. This PR is here for others to use, and I'll submit the v2 version once I've migrated my whole plugin system to it.

Issue for this PR

Related: #32721.

Stacked on #54457 (atomic placeholder deletes), whose commit is included; the change in this PR is the last commit, feat(tui): grouped prompt undo that survives submits and keeps pasted text.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

input_undo runs the textarea's own history, which steps back one character at a time, so taking back a paragraph takes dozens of presses. It knows nothing of the prompt's parts: undoing the deletion of a [Pasted ~N lines] placeholder brings its label back as plain text, and the prompt then sends the label instead of the pasted text. A submit clears it, so text lost to a stray submit or ctrl+u can only be found again through history.

The prompt now keeps its own history (src/prompt/undo.ts) and binds input.undo / input.redo to it while focused:

• Typing is grouped by word: a group ends where a new word starts, after a pause over a second, when the caret moves, or between typing and deleting. Keys that arrive in one read still count as typing.
• A paste, a word or line delete, a newline, clearing, recalling history and a submit are each one step.
• Every entry records the text, the parts with their pasted content, the mode and the caret, and undo restores them together.
• The history lives outside the prompt component, so after a submit undo brings back what was sent, even once the route has moved to the new session, without touching the running turn. Redo empties the prompt again.

prompt.undo_depth sets how many steps are kept (default 500). The default keys are unchanged; keybinds.mdx shows how to move suspend off ctrl+z and use ctrl+z / ctrl+shift+z:

{
  "keybinds": {
    "terminal_suspend": "none",
    "input_undo": "ctrl+z,ctrl+-,super+z",
    "input_redo": "ctrl+shift+z,ctrl+.,super+shift+z"
  }
}

How did you verify your code works?

• test/prompt/undo.test.ts: word grouping, the pause, newlines, line deletes, pastes, typing vs deleting, caret moves, redo, the depth limit, a submit with its placeholder parts, and keys arriving together.
• test/component/prompt-undo.test.tsx drives the real TUI through stdin: ctrl+u then undo word by word; undo after deleting a placeholder sends the pasted text, not the label; undo after a submit brings the text back without an abort request. All three fail on the textarea's own undo.
• bun typecheck and bun test in packages/tui.
• In a real terminal through tmux with ctrl+z / ctrl+shift+z configured as above: hello brave world, ctrl+u, then undo gave back hello brave world, hello brave, hello; after submitting while a turn streamed, undo restored the prompt and the turn kept streaming.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

This PR is AI generated, but under direct supervision and on request of @Nowaker.

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The following comment was made by an LLM, it may be inaccurate: