feat: implement Open Plugins hooks support

[NTaylorMullen]

Summary

Implement support for Open Plugins hooks in Gemini CLI. This allows plugins to intercept and modify core Gemini events (like tool calls, prompts, and model interactions) using the standard Open Plugin protocol.

Details

• Hook Discovery: Automatically discovers hooks from plugin.json or hooks/hooks.json at the plugin root.
• Protocol Translation: Implements OpenPluginTranslator to map internal Gemini events (e.g., BeforeTool) to Open Plugin events (e.g., onTool).
• Variable Expansion: Supports ${PLUGIN_ROOT} expansion in hook commands.
• Environment Injection: Injects PLUGIN_ROOT into the environment for hook child processes.
• Metadata Alignment: Updated ExtensionConfig and GeminiCLIExtension interfaces to include standard Open Plugin fields (repository, homepage, logo, keywords).
• Type Safety: Refactored manifest parsing and hook resolution for strict TypeScript compliance and ESLint alignment.

Related Issues

Fixes https://github.com/google-gemini/maintainers-gemini-cli/issues/1593

How to Validate

1. Run the new unit tests:

   npm test -w @google/gemini-cli -- src/config/open-plugin-discovery.test.ts
   npm test -w @google/gemini-cli-core -- src/hooks/openPluginTranslator.test.ts

2. Run a full build to ensure no regressions:

   npm run build

3. (Manual) Create a plugin with a hooks/hooks.json and verify the hook is triggered during the corresponding event.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed) - Interfaces updated to support metadata.
• Added/updated tests (if needed) - Added openPluginTranslator.test.ts and updated open-plugin-discovery.test.ts.
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt

[github-actions]

🧠 Model Steering Guidance

This PR modifies files that affect the model's behavior (prompts, tools, or instructions).

• ⚠️ Consider adding Evals: No behavioral evaluations (evals/*.eval.ts) were added or updated in this PR. Consider adding a test case to verify the new behavior and prevent regressions.
• 🚀 Maintainer Reminder: Please ensure that these changes do not regress results on benchmark evals before merging.

---

This is an automated guidance message triggered by steering logic signatures.

[gemini-code-assist]

Summary of Changes

Hello, 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 comprehensive support for Open Plugins within the Gemini CLI, significantly enhancing its extensibility. It allows external plugins to define and register hooks that can react to and influence various core CLI operations, such as tool execution and model interactions. The changes involve a new manifest discovery mechanism, event translation, and robust variable expansion, ensuring seamless integration and improved type safety for both existing Gemini extensions and new Open Plugins.

Highlights

• Open Plugin Hooks Support: Implemented support for Open Plugins hooks, allowing plugins to intercept and modify core Gemini events like tool calls, prompts, and model interactions using the standard Open Plugin protocol.
• Hook Discovery and Translation: Enabled automatic discovery of hooks from plugin.json or hooks/hooks.json and introduced OpenPluginTranslator to map internal Gemini events to Open Plugin events.
• Variable Expansion and Environment Injection: Added support for ${PLUGIN_ROOT} expansion in hook commands and injected PLUGIN_ROOT into the environment for hook child processes.
• Metadata Alignment and Type Safety: Updated ExtensionConfig and GeminiCLIExtension interfaces to include standard Open Plugin fields and refactored manifest parsing for strict TypeScript compliance and ESLint alignment.
• Enhanced Extension Loading Logic: Refactored the extension manager to bifurcate loading based on manifest type (Gemini CLI or Open Plugin) and improved error handling for missing configurations.

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