`parseFrontmatter` runs a real YAML parser, so `tools: [read, bash]`
arrives as an array while `tools: read, bash` arrives as a string. The
loader only handled the string form and called `.split(",")`, which
throws on the array form.
The throw escapes `loadAgentsFromDir` -- only `readdirSync` and
`readFileSync` are guarded -- so one agent file using the array form
broke discovery for every agent in that scope, not just itself.
Normalize both spellings in one helper, and type the frontmatter fields
as `unknown` so the string assumptions are explicit. The previous
`parseFrontmatter<Record<string, string>>` annotation asserted a shape
the YAML parser never guarantees, which is what hid the mismatch from
tsc while `AgentConfig.tools` was already typed `string[]`.
