OpenCode DevContainer Feature: Add Gemini Authentication Plugin

[iamralch]

1. High-Level Summary

What changed:
This PR enhances the OpenCode devcontainer feature by integrating the
opencode-gemini-auth plugin. It adds a configuration file, an onCreate
lifecycle hook, updates feature dependencies, and installs the authentication
plugin globally via npm.

Why it changed:
To enable Gemini authentication support out-of-the-box when using the OpenCode
devcontainer feature, providing a smoother developer experience for users who
need Gemini API integration.

2. Detailed Technical Breakdown

2.1 Behavior & Execution Flow

After this change, the OpenCode devcontainer feature:

1. Installs after both common-utils and node features (ensuring npm is
   available)
2. During installation, installs opencode-gemini-auth globally via npm
3. Triggers hooks/on-create.sh when the devcontainer is created
4. Configures OpenCode to use the opencode-gemini-auth plugin via the new
   opencode.json configuration file

2.2 Key Modifications

File	Change
config/opencode.json	New configuration file enabling the gemini-auth plugin
devcontainer-feature.json	Added Node.js dependency and onCreate hook
hooks/on-create.sh	New lifecycle hook script (placeholder)
install.sh	Added npm install for opencode-gemini-auth

2.3 Design Considerations

• Dependency ordering: The feature now explicitly depends on the Node.js
  feature to ensure npm is available before attempting plugin installation
• Global npm install: The plugin is installed globally to make it available
  system-wide within the container
• Lifecycle hooks: Using onCreateCommand allows for future extensibility
  of container initialization logic
• Configuration-driven plugins: The opencode.json file follows OpenCode's
  plugin system, keeping configuration declarative

2.4 Assumptions & Edge Cases

• Assumes Node.js feature is available: If the Node.js feature fails to
  install, the npm command will fail
• Network dependency: Plugin installation requires network access during
  container build
• Whitespace change: The install.sh indentation was changed from spaces
  to tabs, which is a style inconsistency to note
• Placeholder hook: The on-create.sh script currently only echoes a
  message and may need actual implementation

3. Impact Assessment

• Functionality: Adds Gemini authentication capability to OpenCode
  installations within devcontainers
• Performance: Minimal impact; adds one npm package install during build
  time
• Reliability: Introduces a new external dependency (opencode-gemini-auth)
  which could fail if the package is unavailable or npm registry is unreachable
• Security: The gemini-auth plugin will handle authentication credentials;
  users should verify the plugin's security practices
• Developer experience: Improves DX by pre-configuring Gemini authentication
  automatically
• Maintainability: The onCreate hook provides a clean extension point for
  future initialization logic