Customizable prompt

[cdprice02]

The prompt is currently hardcoded as 🦀> in Shell::prefix(). Make the prompt configurable.

Proposal:

• Support format tokens: current working directory, last command's exit code, username, hostname
• Default remains 🦀> for personality
• Consider a FERRISH_PROMPT environment variable or config file (~/.ferrishrc)
• Store prompt config in ShellCtx

Acceptance criteria:

• Prompt is configurable via env var or config
• Format tokens are expanded correctly
• Default prompt unchanged for new users
• Tests verify custom prompt rendering