Review follow-up to #3549.
An envelope is a third-party file, so interpolating source_provider raw let a
provider string carrying a newline close the scalar and inject arbitrary
frontmatter keys. title: on the line above was already quoted; source: now
matches it.
memvelope_conversation_id emitted the literal string undefined when a
conversation carried no id, which asserts a value rather than reporting
absence: every id-less conversation claims the same id, so anything grouping
or deduping on that key merges unrelated pages. The key is now omitted.
Filename and frontmatter read one hasId predicate so they cannot disagree
about whether an id exists. Tests add both cases; the injection case parses
emitted frontmatter with js-yaml rather than substring-matching it.
