Feature Lifecycle Management (Alpha, Beta, GA)

To improve transparency and stability, Gemini CLI needs a standardized way to manage feature maturity. This epic introduces a "Feature Gate" system, allowing features to progress through `Alpha`, `Beta`, and `GA` stages with predictable defaults and constraints.

Goals
- Provide users with clear visibility into the stability of experimental features.
- Standardize feature enablement (Alpha: Off by default; Beta: On by default; GA: Always On).
- Implement a searchable inventory of all available features via the CLI.
- Link features to their respective GitHub tracking issues for feedback.

Prototype is in this branch: https://github.com/google-gemini/gemini-cli/tree/feat/feature-lifecycle