parseMdFile now matches gray-matter (used by OpenCode) for file shapes
OpenChamber previously failed to parse: frontmatter whose closing '---'
sits at end-of-file without a trailing newline, a UTF-8 BOM prefix, and
YAML with unquoted colons in scalar values (via the same sanitizer
OpenCode applies). OpenCode parses these files, so OpenChamber must
too: otherwise the whole file was treated as the prompt body and a
save rewrote the existing YAML block into the body, prepending a
duplicate frontmatter block.
Refs OPE-178
Serhii Dziupin
ff5814a731