- Bump MAM_VERSION to 4.0.0 in lib.sh
- Bump all 8 skill frontmatter versions to 4.0.0 in 3-way lockstep
- Update VERSIONS.md with v4.0.0 changelog, migration guide (B-1..B-5), and skill matrix
- Archive multi-agent version upgrade consensus and peer review reports (439 passed, 0 failures)