feat: add modular Gemini CLI extension support with on-demand skill loading

[gladkia]

This PR refactors the Gemini CLI support to align with its native modular skill system. Instead of aggregating all instructions into a single large file, the build process now generates a standard
Gemini CLI extension that supports on-demand skill activation.

Key Changes:

• Modular Build Process: Each skill is isolated in its own directory with its original YAML frontmatter preserved. References (Markdown/JSON) and scripts are bundled per skill.
• Improved Installation: The global installation path follows standard extension practices (~/.gemini/extensions/wordpress-agent-skills/).
• Context Optimization: Skills are loaded individually by the agent, reducing context usage and improving accuracy.
• Documentation: Updated packaging.md with new build and installation instructions.

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