add support for gemini cli (along with agy) gemini is still supported in enterprise

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Add Gemini CLI Support

Description

Add support for the Gemini CLI alongside the existing Agy integration.

Although Gemini availability has changed for some users, Gemini CLI continues to be supported for Enterprise users. Supporting it would allow Enterprise customers to use Gemini as an alternative CLI provider without losing functionality.

Proposed Changes

• Add Gemini CLI as a supported CLI backend alongside Agy to resume coding/debugging sessions.
• Detect and invoke the Gemini CLI from the sessions CLI.

Motivation

Supporting multiple CLI backends gives users more flexibility and allows Enterprise customers who have access to Gemini CLI to continue using it seamlessly.

Acceptance Criteria

• Gemini CLI can be configured and used alongside Agy.
• Existing Agy functionality remains unchanged.
• Documentation includes setup instructions and any Enterprise prerequisites.