refactor(creds): extract load_or_login_hint helper

[I-am-nothing]

Summary

• New `credentials::load_or_login_hint()` wraps `load()` with the auth-required-command idiom: `NotFound` → "run `mirrorstack login`" hint instead of the raw error.
• `whoami` and `module init` now call `load_or_login_hint()?` (a single line) instead of duplicating the 5-line NotFound match.
• Test extends `credentials_lifecycle` to cover both the missing-file (returns hint) and present-file (returns same data as `load()`) paths. Lifecycle test is single-test by design — env-var mutation in `with_temp_config_dir` would race if split.

Test plan

• `cargo fmt --check`
• `cargo clippy --all-targets -- -D warnings`
• `cargo test` — 22/22

Closes #13