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/utils/agent/agent-loop.ts

@@ -32,6 +32,7 @@ export type CommandConfirmation = {
   review: ReviewDecision;
   applyPatch?: ApplyPatchCommand | undefined;
   customDenyMessage?: string;
+  explanation?: string;
 };
 
 const alreadyProcessedResponses = new Set();