feat: shell command explanation option (#173)

# Shell Command Explanation Option

## Description
This PR adds an option to explain shell commands when the user is
prompted to approve them (Fixes #110). When reviewing a shell command,
users can now select "Explain this command" to get a detailed
explanation of what the command does before deciding whether to approve
or reject it.

## Changes
- Added a new "EXPLAIN" option to the `ReviewDecision` enum
- Updated the command review UI to include an "Explain this command (x)"
option
- Implemented the logic to send the command to the LLM for explanation
using the same model as the agent
- Added a display for the explanation in the command review UI
- Updated all relevant components to pass the explanation through the
component tree

## Benefits
- Improves user understanding of shell commands before approving them
- Reduces the risk of approving potentially harmful commands
- Enhances the educational aspect of the tool, helping users learn about
shell commands
- Maintains the same workflow with minimal UI changes

## Testing
- Manually tested the explanation feature with various shell commands
- Verified that the explanation is displayed correctly in the UI
- Confirmed that the user can still approve or reject the command after
viewing the explanation

## Screenshots

![improved_shell_explanation_demo](https://github.com/user-attachments/assets/05923481-29db-4eba-9cc6-5e92301d2be0)


## Additional Notes
The explanation is generated using the same model as the agent, ensuring
consistency in the quality and style of explanations.

---------

Signed-off-by: crazywolf132 <crazywolf132@gmail.com>

codex-cli/src/components/chat/terminal-chat-input.tsx

@@ -33,6 +33,7 @@ export default function TerminalChatInput({
   loading,
   submitInput,
   confirmationPrompt,
+  explanation,
   submitConfirmation,
   setLastResponseId,
   setItems,
@@ -48,6 +49,7 @@ export default function TerminalChatInput({
   loading: boolean;
   submitInput: (input: Array<ResponseInputItem>) => void;
   confirmationPrompt: React.ReactNode | null;
+  explanation?: string;
   submitConfirmation: (
     decision: ReviewDecision,
     customDenyMessage?: string,
@@ -280,6 +282,7 @@ export default function TerminalChatInput({
       <TerminalChatCommandReview
         confirmationPrompt={confirmationPrompt}
         onReviewCommand={submitConfirmation}
+        explanation={explanation}
       />
     );
   }