docs(go): Add GEMINI.md instructions

[hugoaguirre]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Summary of Changes

Hello @hugoaguirre, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a foundational GEMINI.md document within the Go module, establishing a comprehensive set of development guidelines. The primary goal is to standardize coding practices, ensure high code quality, and enforce strict parity with the canonical JavaScript implementation in terms of API and behavior. These guidelines will serve as a critical reference for all future Go development, promoting consistency and maintainability.

Highlights

• New Go Development Guidelines: A new GEMINI.md file has been added to the go/ directory, providing comprehensive development guidelines for the Go codebase. These guidelines cover various aspects from architecture to testing.
• API and Behavior Parity with JavaScript: The guidelines strongly emphasize that the Go implementation must maintain identical API structure and runtime behavior to the JavaScript (canonical) implementation, ensuring consistency across languages.
• Structured Code Organization: Detailed instructions are provided for package structure (genkit, ai, core, plugins, internal), the use of DefineX for registration and NewX for un-registered component creation, and the functional options pattern for configuration.
• Code Quality and Testing Standards: The document outlines strict standards for code quality, including linting, formatting, strict typing, and comprehensive testing using the standard testing package and github.com/google/go-cmp/cmp for complex comparisons, explicitly discouraging external assertion libraries for new core code.
• Documentation and Generated Files: Guidelines for writing comprehensive GoDoc-style comments, explaining concepts, and providing examples are included. It also explicitly states that generated files (like go/ai/gen.go) should not be modified directly.

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive set of Go development guidelines in go/GEMINI.md. The document is well-structured and covers key areas such as architecture, code quality, testing, and contribution practices. My review includes a couple of suggestions to enhance the clarity of these guidelines, specifically regarding the use of external testing libraries and the placeholder for the copyright year in the license header example.