Centralize shell configuration in ShellCtx

[cdprice02]

ShellCtx in src/ctx.rs currently holds only home_dir and cwd. As ferrish grows (prompt customization, history settings, environment variables), shell state will scatter across modules.

Proposal: Introduce a ShellConfig (or expand ShellCtx) to serve as the single source of truth for all configurable shell behavior:

• Prompt format string
• History file path
• Max history size
• Any future toggle-able behaviors

This keeps state management centralized and testable, per the conventions in CLAUDE.md.