Same fix as d670de2 (cli-mode), applied to:
- openspec/specs/merge-pull-request/spec.md
- openspec/specs/pr-review-write/spec.md
Both kept change-style `## ADDED Requirements` headers that
openspec validate rejects on finalized specs. Replace with
`## Purpose` (new, capability-level summary) and `## Requirements`.
No requirement or scenario content changed.
Closes forgejo-mcp-a0p, forgejo-mcp-517.
84c94fa72a