Unit tests for parsing/discovery

[johnnyhuy]

Summary

• Add unit tests for parsing and discovery.

Context

• Unit coverage protects spec correctness and expected discovery behavior.

Scope

• Frontmatter validation tests (name/description/compatibility).
• Metadata and allowed-tools validation tests.
• Discovery path tests with fixtures.

Out of scope

• Network or integration tests.

Acceptance criteria

• Tests cover happy path and common error cases.