fix: preserve file content when updating ADR metadata (#187)

`set_status()`, `link()`, and `supersede()` previously re-rendered the
entire file through the template engine, destroying custom sections,
markdown links, YAML comments, and MADR structure. Add
`update_metadata()` that surgically edits only status/links/tags fields
in-place via regex replacement for frontmatter files and section-targeted
replacement for legacy files. Also fix `Repository::open()` to respect
the config's template format.

Author: joshrotenberg