fix(skills): read block scalar descriptions instead of indexing the > header

[Max17190]

Why

SKILL.md and prompt-template frontmatter read description: as one line. A YAML block scalar (description: > followed by indented lines, the multi-line spelling third-party skill packages ship) therefore indexed as the single character >, and openmax --check reported the skill as ok: the model saw a name with no "when", and the check that exists to explain such gaps said nothing. Observed live: a third-party skill in this repo's .agents/skills/ rendered in the frozen prompt as - gh-stack: > — .agents/skills/gh-stack/SKILL.md.

Summary

• One description parser (skills::frontmatter_description) shared by SKILL.md and prompt templates: bare and double-quoted values as before; >/| block scalars (with the usual chomping and indentation indicators) fold to the one line the index carries and stop at the next key.
• --check names an empty description as empty (a bare key, or a block header with nothing under it) rather than as absent.

Test Plan

• New block_scalar_descriptions_fold_to_one_line: folded, literal, chomped, and indented headers; a header with nothing under it; a > inside an ordinary value. Fails with block headers read as their literal spelling.
• skill_descriptions_that_the_index_cannot_carry_are_named extended with a hollow block header (warns "an empty") and a folded description (healthy).
• Confirmed on the built binary that the live skill above now indexes with its folded description.
• cargo test --workspace green; clippy at zero warnings.

Greptile Summary

This change adds shared support for multi-line skill and prompt-template descriptions. Explicit indentation is enforced, implicit indentation is derived from the first content line, and duplicate descriptions retain each surface’s established selection behavior.

Confidence Score: 5/5

No blocking failure remains.

Executed parser checks confirmed that malformed scalar indentation is excluded and that skills and templates retain their respective duplicate-description behavior.

T-Rex Logs

What T-Rex did

• I ran an external Rust harness against the real skills::parse_skill_source and templates::parse_template_source entry points, plus the repository’s focused block-scalar unit test.
• The parent commit previously folded two-space and deeper lines, but the current commit returns only "four-space".
• The current commit updates the frontmatter handling by excluding two-space content from description: >4, accepting six-space content, and selecting the last duplicate description for skills and the first duplicate description for templates.
• The focused unit test passed, confirming the corrected behavior is covered.

_{Ran code and verified through T-Rex}

_{Reviews (3): Last reviewed commit: "fix(skills): infer block scalar indentat..." | Re-trigger Greptile}