M5: Per-project generated files (env, kas fragment, logs)

[koenkooi]

Part of #72.

env-<name>.sh (exporting the selector, using the ${VAR:-…} guard idiom so it cannot clobber a shell that set it first — the item-25 regression), macos-<name>.yml as the canonical fragment, and logs/<name>/, which absorbs the item-13 mackas dump retrofit (dumps currently land flat, distinguished only by timestamp). clean's log wipe must scope to the project's own log dir.

Size: M. Risk: medium.