Feedback: new change should emit skip_specs when the schema declares no specs artifact #1638

Version: 1.8.0

A custom workflow schema that declares no specs artifact (only proposal and tasks, for example) produces changes that fail validation the moment they are scaffolded.

Repro:

1. Use a schema whose artifacts list contains no artifact whose 'generates' path lives under specs/.
2. openspec new change my-change
3. openspec validate my-change --type change

Result: 'Change must have at least one delta. No deltas found.' The generated .openspec.yaml contains only 'schema:' and 'created:'.

Such a schema can never produce spec deltas, so every change created under it needs skip_specs: true added by hand before it will validate. In a repo using one of these schemas, that is every change, forever, and it is easy to forget - the failure appears later at validate time rather than at creation.

Suggestion: have 'openspec new change' emit skip_specs: true when the resolved schema declares no artifact whose 'generates' path is under specs/. That condition is already derivable from the schema the CLI parses, and the artifact-graph code already treats that path prefix as the contract custom schemas inherit.

Secondary issue that makes this harder to work around: SchemaYamlSchema is not .strict(), so adding 'skip_specs: true' to schema.yaml is silently accepted and silently ignored. It reads as configured while having no effect. Either honouring it there, or rejecting the unknown key, would be clearer than dropping it.