v0.30.0

• Keep a null _secret value null instead of masking it to ***, so an unset credential no longer reads as a configured one.
• Let a CLI registry declare exit codes beyond 0/1/2 and render them into its --docs reference.
• Write whole sequences and mappings into YAML, including Markdown frontmatter, keeping every byte outside the edited value.
• Extend keyed-list add and remove to YAML frontmatter, and to sequences that are empty or hold a single element.