Reject malformed YAML agent identities before package success #531

🤖 Generated by the Agentic Engineer

Bundled agents can pass the complete packaging validator with frontmatter that a YAML reader cannot load. This leaves a green package capable of failing during native agent discovery.

Observed at current main 088e543663ef310bb59b046d4d70a4a7aa8c14d5: an isolated package copy with refreshed generated resource digests admitted 17 malformed identity values. Examples include name: alpha: beta, a reserved leading @, repeated block chomping flags (|++), a zero indentation indicator (|0) and a declared indentation level that the content does not meet. The complete manifest validator exited successfully for each; an independent YAML decoder rejected each. Valid plain and quoted names and valid block indicator orders remained accepted.

The minimum frontmatter observer checks nonempty scalar content but does not establish that plain scalar grammar and block headers/indentation are valid. Agent files have no later skill-spec gate to catch these cases.

Acceptance:

• Preserve a regression through the complete package validation entrypoint that fails before the repair and rejects all malformed scalar/header cases afterward.
• Retain valid text-valued agent identities, including quoted punctuation and supported literal/folded block forms.
• Keep skill provenance observation and bundled resources valid without claiming to implement every YAML feature.
• Deliver the change through current-head review, required checks and merge.

Part of #436.