Previously, calling `mark-complete` on already-completed requirements
reported them as `not_found`, since the regex only matched unchecked
`[ ]` checkboxes and `Pending` table cells.
Now detects `[x]` checkboxes and `Complete` table cells and returns
them in a new `already_complete` array instead of `not_found`.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>