feat: Add vendor-agnostic Agent Skills (.agents/skills/) and update scripts

[mihaamiharu]

Description

Currently, this repository provides skill definitions in .claude/skills/ for Claude. However, other AI agents (like Gemini CLI) follow the vendor-agnostic standard proposed by agentskills.io, which looks for skills in the .agents/skills/ directory at the project root.
To make this repository "AI agent-ready" across different tools, we should support the .agents/ standard alongside the existing .claude/ one.

Proposed Changes

• Add .agents/skills/: Include the vendor-agnostic directory to ensure all standards-compliant agents can discover the skills out-of-the-box.
• Update scripts/update.js: Modify the internal maintenance script to recognize and prioritize the .agents/ directory when syncing local skills.
• Update README.md: Add a brief section on how to use these skills with Gemini CLI and other agents.

Benefits

• Broad Compatibility: Enables immediate support for Gemini CLI and any other agent following the .agents/ standard.
• Vendor-Agnostic: Reduces reliance on provider-specific directory structures.
• Alignment with Upstream: This change prepares the repository for a planned update to the core Playwright CLI install --skills command.