Document that include selectors only match top-level YAML fields

[Copilot]

Closes:

The -s flag for frontmatter selectors only matches top-level YAML fields, not nested structures. This limitation was not documented.

Changes

Added clarification in four locations:

• Features section: Inline note on "Frontmatter Filtering" feature
• Options reference: Note under -s flag usage
• How It Works: Example in filtering step showing language: go works, nested paths don't
• Rule Files section: Explicit callout with ✅/❌ examples and workaround guidance

Example of the limitation:

---
language: Go        # ✅ Matches: -s language=Go
metadata:
  version: 1.0      # ❌ Cannot match: -s metadata.version=1.0
---

Users needing nested filtering should flatten their frontmatter structure to top-level fields only.

Original prompt

The current includes can only match top-level YAML fields. Highlight that in the docs.

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