[Feat]: Rename AgentCard.Skill to AgentCard.Expertise

### Problem:
The term AgentSkills used in the A2A Agent Card is confusing for users.

### Context:
In the wider community, "Agent Skills" usually refers to skill.md files (as seen on [skills.sh](https://www.skills.sh/) and [openagentskill.com](https://www.openagentskill.com/)). However, A2A uses AgentSkills to mean system capabilities.

### Proposed Solution:

Rename the `Skill` parameter to `Expertise` or `Abilities`.

#### Words to Avoid:
* Word `Capabilities ` is already a reserved word for A2A. https://a2a-protocol.org/latest/specification/#443-agentcard
* Word `Function` is a common word is programming languages.
* Word `Skills` is confusing to use here in A2A.

**Benefit:**
This change will fix the current user confusion and make A2A easier to understand alongside other tools.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Find the AgentCard specification in the codebase, likely in a documentation or schema file. Locate the 'Skill' parameter and rename it to 'Expertise' or 'Abilities', ensuring consistency across all references. Verify the change does not conflict with reserved terms like 'Capabilities' and update any related documentation or examples.

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