Frontmatter we write is not valid YAML when a value contains a colon

[willkg]

What happens

A page title containing : is written to frontmatter unquoted:

---
title: Deploy Runbook: Part 2
page_id: 123
---

That is not valid YAML. VSCode's YAML extension flags it, and any other tool that reads these files with a real YAML parser fails on it — yq, a linter, a static-site build over the same tree, another editor's frontmatter plugin. gopkg.in/yaml.v3 reports mapping values are not allowed in this context.

markfluence itself reads it back correctly, which is why this has gone unnoticed: frontmatter.Extract splits each line at the first : only, so title: a: b round-trips as a: b for us and breaks for everyone else. There is a test pinning the current output — internal/frontmatter/frontmatter_test.go, {"colon value bare", "title", "a: b", "", "title: a: b"}.

Why it happens

renderValue in internal/frontmatter/frontmatter.go decides whether to quote by asking "would our own ParseValue round-trip this?" rather than "is this valid YAML?". A colon round-trips through our parser, so it is never quoted.

The colon is one member of a larger class

The same predicate lets all of these through unquoted, and each is either invalid YAML or reads back as the wrong type:

written	a real YAML parser sees
title: a: b	parse error
title: true	boolean true, not the string
title: 123	integer 123
title: null / title: ~	null
title: no	boolean false
title: [draft] Foo	flow sequence
title: @home / *star / &anchor / %pct / - dash / `	pipe`

parent has the same exposure, since it can hold a relative .md path.

Scope

markfluence has no YAML dependency today — internal/frontmatter is a hand-rolled flat-key parser. Fixing this by widening the quoting predicate means hand-maintaining a YAML plain-scalar rule set in a package with no YAML parser to check itself against, which is how the version that forgets % ships. The alternative is to adopt a real YAML library and let it decide quoting.

Related: #100 (markfluence.yaml project-wide settings) needs a real YAML parser anyway.