feat: add mode-change hook for automatic IME switching

[aizigao]

Run an external shell command on every NORMAL <-> INSERT transition so users can switch input methods automatically (e.g. Rime <-> ABC via im-select), preventing Chinese IME from interfering with vim navigation.

• Add piVim.modeChange config (insert/normal keys) read from ~/.pi/agent/settings.json or project .
• Emit pi-vim:mode-change on pi.events for other extensions to subscribe
• Update README.md with the configure section

in pi settings

"piVim": {
  "modeChange": {
    "insert": "im-select im.rime.inputmethod.Squirrel.Hans",
    "normal": "im-select com.apple.keylayout.ABC"
  }
}

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Most of the code is written in Claude Opus with Pi coding agent