Allow rootless mere store init #85

`mere store init` gated on uid 0 and unconditionally chowned every
directory to root:root. With a user-writable MERE_ROOT this gating
contributed nothing (filesystem permissions already control access)
but it blocked rootless installs entirely.
- Drop the uid 0 check in `init.initialize()`. Filesystem permissions
 gate access: if you can write to the prefix, init succeeds; if not,
 the filesystem returns EACCES at the failing operation.
- Drop `setOwnership()` calls from `fixDirectory()`. Files inherit
 ownership from whoever invoked `mere store init`: root if run with
 sudo, the invoking user otherwise.
- Stop flagging `wrong_ownership` in `checkDirectory()`. Ownership is
 informational only; mere does not manage it from this code path.
- Update --root help text to clarify it is the install prefix and the
 store lands at <prefix>/mere/.
Verified end-to-end against /tmp/mere-test as a non-root user:
- `mere --root /tmp/mere-test store init` creates 18 user-owned dirs
- `mere --root /tmp/mere-test profile create test` succeeds
- `mere --root /tmp/mere-test install -p test busybox` downloads,
 signature-verifies, extracts to store, materializes profile
- Profile binary runs without sudo