feat: configurable keybindings for all editor and app actions

All keybindings configurable via ~/.pi/agent/keybindings.json

Editor actions:
- cursorUp, cursorDown, cursorLeft, cursorRight
- cursorWordLeft, cursorWordRight, cursorLineStart, cursorLineEnd
- deleteCharBackward, deleteCharForward, deleteWordBackward
- deleteToLineStart, deleteToLineEnd
- newLine, submit, tab
- selectUp, selectDown, selectConfirm, selectCancel

App actions:
- interrupt, clear, exit, suspend
- cycleThinkingLevel, cycleModelForward, cycleModelBackward
- selectModel, expandTools, toggleThinking, externalEditor

Also adds support for numpad Enter key (Kitty protocol codepoint 57414
and SS3 M sequence)

Example emacs-style keybindings.json:
{
  "cursorUp": ["up", "ctrl+p"],
  "cursorDown": ["down", "ctrl+n"],
  "cursorLeft": ["left", "ctrl+b"],
  "cursorRight": ["right", "ctrl+f"],
  "deleteCharForward": ["delete", "ctrl+d"],
  "cycleModelForward": "ctrl+o"
}

packages/tui/src/tui.ts

@@ -5,7 +5,7 @@
 import * as fs from "node:fs";
 import * as os from "node:os";
 import * as path from "node:path";
-import { isShiftCtrlD } from "./keys.js";
+import { matchesKey } from "./keys.js";
 import type { Terminal } from "./terminal.js";
 import { getCapabilities, setCellDimensions } from "./terminal-image.js";
 import { visibleWidth } from "./utils.js";
@@ -146,7 +146,7 @@ export class TUI extends Container {
 		}
 
 		// Global debug key handler (Shift+Ctrl+D)
-		if (isShiftCtrlD(data) && this.onDebug) {
+		if (matchesKey(data, "shift+ctrl+d") && this.onDebug) {
 			this.onDebug();
 			return;
 		}