Artifact signing: issuer-key fallback to device_key may be wrong for multi-key identities

[bordumb]

Summary

The artifact-signing fix in 617378fc ("fix: issuer signature in golden path dropping") is correct and
important — auths sign <file> was producing device-only attestations that failed verification with
"missing issuer signature". The fix makes a file attestation always carry an issuer signature. But the
fallback when no --key is given assumes the device key is the issuer key, which is only true for a
single-key identity.

Where

crates/auths-cli/src/commands/artifact/sign.rs:

// A file attestation must be attributable to the signing identity, so it always
// carries an issuer signature: use the explicit `--key` when given, otherwise the
// same key that signs as the device (for a single-key identity they are one key).
identity_key: Some(SigningKeyMaterial::Alias(KeyAlias::new_unchecked(
    key.unwrap_or(device_key),
))),

The concern

For a single-key identity this is correct (device and issuer are the same key — the comment says so,
and the new test_golden_path_artifact_sign_verify covers it). For a multi-key identity, falling back
to device_key as the issuer signer may be wrong:

• the device key may not be the authoritative issuer key for that identity, so the attestation's "issuer
  signature" could be produced by the wrong key; and/or
• it collapses issuer == device when they are meant to be distinct, which a verifier may (or may not)
  accept depending on how the issuer slot is checked.

Ask

Confirm the multi-key path, then pick one:

1. It's fine — multi-key identities always pass --key, and that's enforced/documented. Add a test
   that asserts the fallback is only reachable for single-key identities (or that sign errors clearly if
   no --key is given on a multi-key identity).
2. It's wrong — the fallback should resolve the identity's actual issuer key rather than assume
   device_key. Add a multi-key sign → verify test.

Severity

Low–medium. The single-key default works and is regression-tested; this only matters if artifact signing
is relied on for multi-key identities without an explicit --key. Worth a glance before that path is
trusted.