fix(harness): stop gen-subagents from parsing registry as an agent spec

Parent: #384 (P2).

## Scope
Constrain gen-subagents discovery to agent spec files and exclude registry.yaml by schema-aware selection.

## Acceptance
- Generation emits no registry missing-name warning.
- All real agent specs still generate both supported formats.
- A regression test includes registry.yaml beside agent specs.

Delivery discipline: one issue, one worker branch, one PR.

Start by tracing gen-subagents discovery and the existing generation tests, then inspect how registry.yaml is selected alongside agent spec files. Add a regression case with registry.yaml beside real specs; done means no registry missing-name warning and both supported formats still generate for every real agent spec.

Written by the indexing model from the issue text.