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/test/key-tester.ts

@@ -1,5 +1,5 @@
 #!/usr/bin/env node
-import { isCtrlC } from "../src/keys.js";
+import { matchesKey } from "../src/keys.js";
 import { ProcessTerminal } from "../src/terminal.js";
 import { type Component, TUI } from "../src/tui.js";
 
@@ -17,7 +17,7 @@ class KeyLogger implements Component {
 
 	handleInput(data: string): void {
 		// Handle Ctrl+C (raw or Kitty protocol) for exit
-		if (isCtrlC(data)) {
+		if (matchesKey(data, "ctrl+c")) {
 			this.tui.stop();
 			console.log("\nExiting...");
 			process.exit(0);