feat(sleep): add opt-in section_contains rule judge

[Bogdan (Dan) Baciu (bogdanbaciu21)]

Summary

Add an opt-in section_contains rule-judge operator for permissive heading checks while preserving existing scoring behavior.

• keeps section_present unchanged
• registers section_contains across scoring, validation, mining, prompts, and backend guidance
• classifies it as a shape-only operator
• documents and tests the strict/permissive distinction

Contract

section_contains performs a literal, Unicode case-insensitive substring match on ATX-heading-shaped lines: up to three leading spaces, one to six # markers, then whitespace or end of line.

It accepts numbered, bilingual, and annotated headings such as ### 1. Key Risks (Риски) — overview, while ordinary body text, bold labels, Setext headings, and blockquoted lines do not match.

Because section_present is unchanged, existing task sets retain their current scores.

Attribution

This implements the additive, compatibility-preserving operator proposed by Koi Lee (@koi-lee) and accepted by maintainers in #175. Thank you, Koi Lee (@koi-lee), for defining the direction and expected behavior.

Validation

• 97 focused judge/miner tests passed
• 1,054 suite tests and 130 subtests passed; 12 skipped
• one current-main Copilot parser regression was deselected after reproducing it unchanged at a17de4e
• python -m mkdocs build --strict passed
• Ruff passed on the focused files; backend.py retains seven pre-existing findings outside this two-line change

Related to #175.