The archived spec.md kept the change-delta format ("## ADDED
Requirements") instead of the conventional spec format, causing
`openspec validate --all --strict` to fail with "Spec must have a
Purpose section". Adds top-level title, ## Purpose, and renames
"## ADDED Requirements" to "## Requirements". Unblocks the Tekton
openspec-validate gate on PR #148.
Refs: forgejo-mcp-1tc