Update notes.md with the v0.5.1 Linux portability baseline (commits
7a7b249, 1033072): WSL-native validation passed, install.sh works,
ctask doctor recognizes the Linux statusline helper, the cross-built
Linux binary is statically linked, and WorkspacePath was removed from
new task.yaml metadata. Add load-bearing notes for the new invariants
(CGO_ENABLED=0, install script does not modify shell config) and a
"Next: v0.5.2" pointer.
Also check in v0.5.2-spec.md so the workspace-retrieval round has the
same on-disk durability as the prior specs.
