Define diagnostics and artifact modeling for pipeline results

Background:
- Pipeline design calls for structured diagnostics and artifacts.

Scope:
- Define Diagnostic and Artifact types with severity, code, message, location, and metadata.
- Ensure data serializes cleanly for JSON output and analyzer use.

Acceptance criteria:
- Diagnostic and Artifact types in pkg/pipeline (or subpackage).
- JSON encoding tests cover severity and location fields.