Add downstream GitHub Actions recipe for AgentV evals

[smota]

Parent epic: #18

Goal

Provide CI recipe for downstream projects to run AgentV evals and preserve result artifacts.

Requirements

• GitHub Actions example installs AgentV.
• Runs agentv eval ./evals/<asset>.eval.yaml.
• Uploads .agentv/results artifact even on failure.
• Documents how adopters can use regression gates/compare once they have baselines.
• Calls out secret handling for provider API keys.

Acceptance criteria

• toolsets/agentv/templates/github-actions-agentv.yml exists.
• README explains how to copy/adapt workflow.
• Workflow avoids committing secrets.
• Workflow treats AgentV as optional adopter CI, not MetaSkills repo default CI.