docs(MIGRATION.md): update base reference after add-claude merge

rajiv/add-claude was merged to feature/darwin upstream, so the doc's
"on top of rajiv/add-claude (PR #45)" reference is stale. The branch
is now rebased onto origin/feature/darwin directly — same diff, just
a fresher base.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rajivml

MIGRATION.md

@@ -376,7 +376,8 @@ These need eyes — automated coverage doesn't catch them:
 
 ## 12. Branch contents at-a-glance
 
-16 commits on top of `rajiv/add-claude` (PR #45):
+17 commits on top of `feature/darwin` (which now includes the merged
+`rajiv/add-claude` work — PR #45):
 
 ```
 [BG-scale] darwin-kubernetes: port split-background manifests + lock convention in AGENTS.md