4.0.0

• Support of SSM as profiles source
• profiler remove and profiler show go through the local repository rather than calling pkg/profile directly, and the local repository's Remove, Save and Show are implemented rather than stubbed.
• profiler show lists a profile's variables in a stable order.
• profiler remove accepts several variables at once, reports a profile that does not exist instead of failing silently, and refuses to remove profile_name on its own.
• Remove the unused top-level profile package. pkg/profile is the single Profile type; Design.md records why.
• Remove dead scaffolding left over from the redesign: repository/local/localProfile.go, the unused repository.Repository struct and its New(), and the never-implemented ComposeProfile and its two helpers.
• profiler use stacks profiles instead of replacing them: a profile applied while another is in use exports both profiles' variables, the newly applied one winning on any variable they share. profiler status reports the whole stack (A+B), and leaving the shell use spawned unstacks. Design.md records the decision.
• profiler use exports profile_keys, naming the variables the profile in use owns. It is what lets the next use stack onto them, and it makes the .profiler file a full record of the environment in use rather than only of the profile applied last.