Verify prerequisite implementation content separately from PR merge status

A merged prerequisite PR can contain only an empty kickoff commit. Before starting a dependent phase, inspect its actual changed files and the remote integration tree, then separately check acceptance evidence. Squash merges add another trap: differing commit ancestry does not prove missing code; equal Git tree IDs prove equal snapshots. Snapshot equality still does not replace verification required for the final base/head. Provenance: direct Git/GitHub prerequisite checks during an implementation preflight; project identifiers, code, and private evidence omitted.