v0.1.1 — workspace housekeeping

Changed:
- `rtb-core` renamed to `rtb-app`. The primary export is the `App`
  context; the old name was abstract and invited mis-reading.
  Every `use rtb_core::…` must be updated to `use rtb_app::…`;
  every Cargo dep renamed from `rtb-core` to `rtb-app`. No API
  signatures changed.
- `rtb-credentials::Resolver` now clones `SecretString` directly on
  the literal-credential path instead of round-tripping through a
  plain `String`, keeping the value inside the zeroize-on-drop
  container for the whole copy.

See CHANGELOG.md § [0.1.1] for migration notes.