Bug: doctor reports an unreadable install state as unowned #437

Summary

doctor --target-dir reports ownership=unowned with exit 0 when the target's .cortex/install-state.json exists but cannot be read.

Reproduction

1. Create a directory target with .cortex/install-state.json set to mode 0000, running as a non-root user.
2. Run cortex doctor --target-dir <dir>.

Expected

An unreadable ownership record is reported as invalid, not as absent. Only a missing file means unowned.

Actual

exit=0 out="target=<dir> presence=present writability=writable ownership=unowned"

Environment

main at 46f2870, macOS, non-root user. As root the file stays readable, so it does not reproduce.

Evidence

internal/cli/doctor.go:169-172 maps every os.ReadFile error, including permission denied, to unowned. Found while verifying #360, #365 and #366. No test on main covers it. Fix: fs.ErrNotExist gives unowned, any other error gives invalid, with a CLI test using chmod 000.