Add demo guidance skill to Agent skills and link skills doc in README

[Karosown]

Motivation

• Provide an explicit demo guidance skill to help users create minimal runnable examples for Client/Agent + tool integration.
• Avoid users relying on outdated demo files by constraining demo guidance to current API conventions.
• Make the Agent skills machine- and human-readable in Claude Skills YAML format for reuse in prompts and docs.
• Improve discoverability by surfacing the skills document from the module README.

Description

• Added a new Claude Skills YAML file ai/agent/skills.md that enumerates Agent capabilities and includes the new Demo 示例引导与最小可运行说明 skill with inputs, outputs, and constraints.
• The demo skill specifies providing example steps or pseudocode and constraints to avoid referencing stale demos and to match current API conventions.
• Updated ai/agent/README.md to add a 技能清单 section that links to skills.md for easy reference.
• Kept the existing skills schema (name, description, input, output, constraints) and appended the demo guidance entry accordingly.

Testing

• No automated tests were run because this change only modifies documentation files.
• Verified that ai/agent/skills.md was created and contains the new demo guidance skill entry.
• Confirmed ai/agent/README.md now includes a 技能清单 link to the skills document.
• No build or runtime impacts expected from documentation-only edits.

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Codex Task