docs-hygiene/audit-noise: scanner exemption gaps — frontmatter unimplemented, marker matches as substring, no code-fence awareness, section-state leaks

[kyle-sexton]

Bundle of scanner-correctness gaps from the plugin-quality audit (packet docs-hygiene-audit-noise/20260815T173035Z), each reproduced:

1. Frontmatter exemption unimplemented (Medium, F2): SKILL.md hard rules exempt frontmatter; detect.sh scans it — YAML description lines produce Tier-1 findings.
2. Opt-out marker matches as bare substring (Medium, F4): prose MENTIONING markdown-discipline-ignore acts as a live marker — SKILL.md's own hard-rules line swallows the following content. Require the marker to be an actual HTML comment line, not a substring.
3. No code-fence/inline-code awareness (Medium, F5): fenced output-schema examples and shape tables self-match; fence-skipping alone removes a large slice of the ~45% false-positive rate observed on durable surfaces in the repo-wide run (PR feat(docs-hygiene): repo-wide audit-noise treatments, clean-tree fallback, paragraph-scope markers #2721).
4. Section-exemption state leaks (Low, F7): exemption toggles only on ## headings — an exempt ## Sources followed by an H1 stays exempt to EOF, and ### Sources is never exempt.
5. Note: the auditor's F3 (CHANGELOG filename exemption) was already fixed in PR feat(docs-hygiene): repo-wide audit-noise treatments, clean-tree fallback, paragraph-scope markers #2721; F6 (contract-root env loss) is folded into the perf issue's fix 1.

Info-level notes riding along (F10): cd-before-target-resolution silently skips relative paths given from another cwd; spaced-filename parsing in the uncommitted-files fallback; inverted return "$found" idiom.