Fix Resend edge rejection of release notification client

[mikkokotila]

The first published plugin release succeeded, but its email job received HTTP 403 from Resend. A read-only authenticated probe reproduces the provider edge rejection (error code: 1010) with Python's default user agent; the same request succeeds with an explicit Resolvent identity and confirms autonom.io is verified.

Set that identity on the email client. The regression simulates the observed rejection and verifies successful acceptance with the original payload, authorization and idempotency key. Preserve the existing pending delivery journal, so the next main release run retries the same notice without changing release assets or sending duplicates.

Compatibility: this script is outside the installed plugin. No manifest, package bytes, protocol or version changes are needed.

Validation: 69 unit tests, repository closure, reproducible host packages and the version rule pass through the local managed runner. PR CI reports its own results. Failure evidence: https://github.com/autonomio/resolvent/actions/runs/37914548629

[bit-mis]

Reviewed: no blocking findings.

[bit-mis]

Reviewed: no blocking findings.

[bit-mis]

Reviewed: no blocking findings.

[mikkokotila]

No code change is requested: all three reviews report no blocking findings on 3287e67. The 69 unit tests, closure, reproducible package and version checks pass, as does current-head CI. Recorded these reviews as non-fix dispositions; GitHub still requires a formal approval for the ordinary merge.