Reject unusable decoded custom-agent identity text #511

🤖 Generated by the Agentic Engineer

Problem and evidence

The package gate accepts a custom agent whose YAML name consists only of an escaped control character. A complete synthetic package passes the actual manifest validator with this unusable agent identity, just as its healthy control does.

Consumers depend on package metadata to present discoverable, identifiable agents. A nonempty encoded scalar does not establish usable text.

Expected outcome

The minimum header observer must reject unusable decoded agent identity and description text while preserving supported visible text and legitimate description blocks. Unsupported scalar observations must remain explicit failures.

Acceptance criteria

• Reproduce acceptance through the actual package validator before implementation.
• Reject control-only and hidden-control identity values in supported quoted forms.
• Preserve normal ASCII and Unicode agent names and visible descriptions.
• Keep repository provenance observation and body exclusion intact.
• Run package and manifest regression suites and verify the normal package path.

Rough size: S. Part of #223.