feat: add gemini agent support

[xunleii]

First of all, thanks for this project! I'm really looking forward to testing it out properly. Since I currently only have access to Gemini (and GitHub Copilot), I decided to implement the Gemini integration myself to get things moving.

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Gemini CLI Provider Integration:

• Added a new GeminiProvider in internal/agent/gemini.go to interface with the Gemini CLI, including methods for command construction, output parsing, and log file naming.
• Implemented detailed unit tests for GeminiProvider in internal/agent/gemini_test.go.
• Updated the agent resolver in internal/agent/resolve.go to support "gemini" as a valid provider and improved error messages for unknown providers.
• Added tests for Gemini resolution in internal/agent/resolve_test.go. [1] [2]

Gemini Stream Event Parsing:

• Introduced internal/loop/gemini_parser.go with a parser for Gemini's stream-json output, mapping Gemini events to Chief's internal event types.
• Added thorough tests for Gemini event parsing in internal/loop/gemini_parser_test.go.

Documentation and Configuration Updates:

• Updated documentation (README.md, docs/reference/configuration.md) and CLI help text to list Gemini as a supported agent provider and clarify configuration options. [1] [2] [3] [4] [5] [6]

These changes make it possible to use the Gemini CLI as a drop-in agent for Chief, with full support for configuration, event handling, and output processing.

Note

Current Status: Full vibe coding mode with a quick review on my end.
Looks solid—it works with Gemini CLI, so I'm good to go. Successfully built a short Python app with it

Fixes #56