feat: add shell command execution support

[aweis89]

Summary

Adds support for executing shell commands directly from the input window by prefixing them with !.

Changes

• Shell command execution: Commands starting with ! are executed using vim.system() with the user's configured shell
• Output display: Command output is shown in the output window with clear formatting
• Context integration: Users are prompted to add command + output to the input context for LLM interaction
• Error handling: Failed commands display error notifications with exit codes
• UI updates: Input placeholder now shows ! shell hint

Behavior

1. User types !<command> in input window
2. Command executes asynchronously using user's shell (vim.o.shell)
3. Output displays in output window: $ <command>\n<output>
4. Prompt asks: "Add command + output to context?"
5. If Yes: Appends formatted message to input buffer, clears output window, focuses input
6. If No: Just focuses input window
7. User can add additional context/prompt before submitting to LLM

Format

Command: `echo test`
Exit code: 0
Output:
\`\`\`
test output
\`\`\`

Testing

• 5 comprehensive unit tests covering all scenarios
• All existing tests still pass

Resolves #147

[sudo-tee]

This is great , thanks a lot for this