fix(skills): parse SKILL.md frontmatter when --- is not the first byte

[eldonm]

Summary

A SKILL.md whose --- is not the very first byte silently loses its entire frontmatter. _parse_frontmatter tested the delimiter on the unstripped text while using the stripped text for everything else:

content = raw.strip()
if not raw.startswith("---"):      # raw, not content
    return {}, content

A leading blank line, an indent, or an editor-inserted UTF-8 BOM therefore reads as "no frontmatter", and the whole block is returned as body.

Related issues

Diagnosed from a field report that skill_only_tools and channel_overrides "were not working", with allowed-tools appearing in the prompt. Neither feature was at fault — see below.

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change
• Documentation
• Chore / tooling / CI

Description

The bug. Two consequences, both silent, and they compound into one confusing symptom:

1. allowed-tools is never read, so SkillDoc.requires_tools is empty and the skill owns nothing. Any tool named in skill_only_tools then has no owning skill and fails closed — the gate looks broken when the declaration simply never loaded.
2. The frontmatter text lands in SkillDoc.body, which is rendered into the prompt's skills section, so allowed-tools: appears verbatim to the model.

The chain is _parse_frontmatter → allowed_tools → SkillDoc.requires_tools → build_skill_gate ownership, and separately content → SkillDoc.body → skills section. One broken parse, both symptoms.

Reproduced with a real four-tool declaration:

Input	tools parsed	frontmatter leaked into body
clean	4	no
leading newline	0	yes
leading spaces	0	yes
UTF-8 BOM	0	yes
CRLF	4	no

The fix. Strip the BOM, then test and split the stripped text. sop_extend.py:138 in this same codebase already does raw.strip().startswith("---") — skill_resolve was the outlier, which is what makes this look like an oversight rather than intent.

Checklist

• I read CONTRIBUTING.md and the local CLAUDE.md.
• pre-commit run --all-files passes.
• pytest tests/ passes — 0 failures.
• Bug fixes cite file:line in the description.
• CHANGELOG.md — happy to add an entry; held off since this restores intended behaviour rather than changing it. Say the word.
• No ADR was modified in place.

Notes for reviewers

Written test-first. Five parametrised leading-character cases (newline, blank lines, spaces, BOM, BOM+newline) confirmed red before the fix, plus three controls so the fix cannot over-reach:

• a clean file still parses
• a --- horizontal rule inside the body is not treated as a delimiter
• a file with no frontmatter is untouched

That middle control matters: split("---", 2) with maxsplit=2 is what keeps a markdown rule in the procedure from being eaten, and it would be easy to break while "fixing" the delimiter check.

Diagnostic tip for anyone hitting this before the fix ships:

head -c 16 path/to/SKILL.md | xxd | head -2

efbbbf at the start is a BOM; 0a is a leading newline. Removing either makes the skill work immediately, which is also the fastest way to confirm this is the cause.

Steps to test

pytest tests/scaffold/test_skill_resolve.py -q