secureflow: Grok Integration

[shivasurya]

This pull request adds support for the new xAI Grok 4 Fast Reasoning model to the SecureFlow extension. The changes update the model lists, type definitions, UI, and documentation to ensure Grok 4 Fast Reasoning is available and correctly displayed throughout the extension.

Support for Grok 4 Fast Reasoning model:

• Added grok-4-fast-reasoning to the supported models list in package.json, including its description for selection in the settings UI. [1] [2]
• Updated the AIModel type definition in settings-manager.ts to include grok-4-fast-reasoning.

UI and display updates:

• Added Grok 4 Fast Reasoning to the model selection dropdown in the webview UI (index.html).
• Updated display name and provider mapping for grok-4-fast-reasoning in ui-helpers.js so it is shown correctly and attributed to xAI. [1] [2]

Documentation:

• Updated the CLI README to include Grok 4 Fast Reasoning under supported models, along with other recent model additions and provider clarifications.