credstore: expose remaining ByteNess backends (kwallet, keyctl, passage) on user ask #30

Context

ByteNess v1.9.3 ships three additional backends that #24 deferred under the "defer obscure ones until asked" rule from the original ticket:

• `kwallet` — KDE Wallet over D-Bus (KDE desktops; complements GNOME's `secret-service`).
• `keyctl` — Linux kernel keyring (per-session in-memory; CI / ephemeral use cases).
• `passage` — modern age-encrypted replacement for `pass`; same opener model.

Trigger

Open a PR when a user (internal or external) asks for any of them. Don't speculatively add all three.

Scope (per backend)

Mirror the `pass` pattern from #26:

• Add `Backend<Name>` constant; append to `allBackends` (memory stays last).
• Update `TestAllBackends_MatchesConstants` expected slice.
• Add `TestSelectBackend<Name>NeverAuto`.
• Update `buildKeyringConfig` only if the backend needs construction extras (`kwallet` needs `KWalletAppID` / `KWalletFolder`; `keyctl` needs `KeyCtlScope` / `KeyCtlPerm`; `passage` is like `pass`).
• Update §1.4 + §1.10 docs.

Out of scope

• 1Password backends — see the sibling follow-up issue gated on the SDK leaving beta.