fix(decision-records): flag strict-YAML hazards in front matter

[trungfinity]

Problem

parse_front_matter is deliberately tolerant (line-based, first-: split), so it accepts front matter that a spec-compliant YAML parser rejects. The sharp edge: an unquoted : inside a plain scalar starts a nested mapping in real YAML, which is illegal mid-value. GitHub's front-matter renderer uses a strict parser, so an affected record displays an error banner instead of rendering — hit in practice by skymavis/lanh-ai's 0009, whose summary contained …two modes: channel mode… ("Error in user YAML: mapping values are not allowed in this context").

Fix

New front_matter_errors() wired into check: for every record and draft, plain-scalar values containing : (or ending with :) are reported as errors, as are unbalanced opening quotes. Quoted values, flow sequences ([...]), null/empty, and folded/literal block scalars are exempt — block-scalar bodies are skipped by indent, mirroring the parser.

Deliberately narrow: this targets the one hazard class with a public failure surface, not full YAML validation — no new dependency.

Testing

• 4 new tests: unquoted colon fails with the expected message; the same colon quoted passes; the same colon in a folded block scalar passes; an unbalanced quote fails.
• Full suite: 66 passed.
• Field-checked against skymavis/lanh-ai: the patched check flags 0009's original summary and passes its quoted fix (skymavis/lanh-ai#19).