[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

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