Validate only the agents selected by explicit component arrays #439

🤖 Generated by the Agentic Engineer

Problem

The portable package contract says an explicit agents array determines selected agents. The catalogue honors that array, but agent validation still scans every Markdown file in agents/, so an unrelated unselected file can reject an otherwise valid selected agent.

Proposed direction

Use the declared selection consistently for agent validation and catalogue resource counting. Retain complete default discovery and reject invalid selected targets.

Acceptance criteria

• A valid explicitly selected agent passes with unrelated unselected Markdown present.
• An empty explicit selection contributes no agent resource.
• A malformed, linked or missing selected target fails.
• Default discovery retains required suffix, frontmatter and resource-count checks.

Size: S. Part of #38.