Fix incorrect Gemini CLI extension command syntax

[cnaples79]

Summary

• Fixes incorrect command syntax in the setup instructions
• Changes gemini extensions add to gemini extensions install
• Addresses issue where the documented command fails on current stable release

Problem

The README documented the following command for installing the MCP server as a Gemini CLI extension:

gemini extensions add --source https://github.com/GoogleCloudPlatform/gemini-cloud-assist-mcp

However, this command fails with the current stable release (v0.4.1) of the Gemini CLI because the add subcommand does not exist.

Solution

Updated the command to use the correct install subcommand:

gemini extensions install --source https://github.com/GoogleCloudPlatform/gemini-cloud-assist-mcp

Test Plan

• Verified the correct command syntax matches the Gemini CLI help output
• Confirmed this change aligns with the current stable release functionality
• Users can now successfully follow the installation instructions

This is a simple documentation fix that ensures users can successfully install the extension using the provided instructions.

Fixes #13

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[kyleabenson]

Hi, the docs here show the following example:
gemini extensions install https://github.com/google-gemini/gemini-cli-security

I think the --source flag can be removed as well

[cnaples79]

Thanks for the feedback! You're absolutely right - I've updated the PR to remove the --source flag as well.

The command now matches the pattern used in other examples:

gemini extensions install https://github.com/GoogleCloudPlatform/gemini-cloud-assist-mcp

This is cleaner and aligns with the established Gemini CLI extension installation syntax.

[cnaples79]

@shreyasnbhat Thanks for the review. I'd love to help more with the project and contribute if there are any other high visibility issues or features.