[Docs] Document .agents/skills directory alias for cross-tool alignment

[kevmoo]

Is your feature request related to a problem? Please describe.
Gemini CLI currently uses .gemini/skills for skill discovery. However, to align with emerging industry standards and provide a more tool-agnostic configuration path, we should explicitly document and support the .agents/skills convention.

As noted in the Antigravity Skills documentation, standardizing on a generic .agents/ path allows for better interoperability and a more intuitive experience for developers working across different AI agent platforms.

Describe the solution you'd like
Update the Gemini CLI documentation to:

1. Officially document .agents/skills/ as a supported directory for both User (~/.agents/skills/) and Workspace (.agents/skills/) tiers.
2. Clarify the precedence rules: when both exist in the same tier, .agents/skills/ is prioritized over .gemini/skills/.
3. Update tutorials and CLI examples to reflect this standardized path as the preferred generic alternative.

Describe alternatives you've considered
Continuing to use only .gemini/ specific paths, which can create friction for users seeking tool-agnostic configurations.

Additional context
Aligning with the .agents/ convention ensures that skills created for Gemini CLI follow the same discovery patterns seen in other modern AI agent tools. Support for this alias was previously added in PR #18151.