fix(packages/config,packages/cli): infer nested-group card descriptions

[zrosenbauer]

Summary

Auto-generated group landing pages render blank nested-group cards when the group has an overview page but no explicit config description.

Expected behavior

Resolve one group description and reuse it for both the parent card and the group's generated landing introduction.

Precedence:

1. Explicit card description.
2. Explicit page or group description.
3. Overview-page frontmatter description.
4. Overview-page first paragraph when inference is enabled.

Acceptance criteria

• Nested auto-derived groups use descriptions from overview.md, index.md, readme.md, or the configured entry-slug page.
• The parent card and generated group landing use the same resolved value.
• Explicit descriptions override inferred values deterministically.
• Users can disable first-paragraph inference without disabling frontmatter descriptions.
• Config schema, public types, generated schema, and documentation agree.
• Tests cover precedence, landing-file priority, disabled inference, and nested groups.

Prior work

Closed draft PR #156 contains the root-cause analysis, implementation spike, and tests.