Replace p10k with simpler custom prompt

Powerlevel10k was flaky when run by agents. Replace with a pure-zsh
prompt that shows similar information:
- Current directory (blue)
- Git branch with staged/unstaged/untracked indicators
- Command execution time (if > 3s)
- Prompt character (green ❯ for success, red for error)
- Time on right side

Author: claude