Verify marketplace release candidates against fresh remote state #266

🤖 Generated by the Agentic Engineer

Problem and evidence

Release preparation and verification are merged in #249 and #253. They inspect local Git objects, so a successful assessment can still use stale tags or nominate a commit that is no longer the default-branch tip. Publishing from that assessment alone would not establish agreement with GitHub.

Proposed outcome

Add an opt-in, read-only prepublication check that combines the existing candidate verifier with fresh GitHub state. Bind the repository identity, current default branch, complete tag inventory and absent candidate release to the selected source and release commits. Reject missing, malformed, incomplete, changed or conflicting observations. Do not modify refs, the checkout, tags, releases or installed plugins.

Acceptance criteria

• The local verifier must pass for the explicit full source and release commits.
• The remote repository identity and current default-branch tip must match the intended repository and release commit.
• Complete remote tag identities must agree with local tags, including annotated tags; a moved baseline, omitted page or occupied candidate tag fails.
• The intended GitHub release must be absent; authentication, transport and malformed-response failures never mean absence.
• Repeat the remote observation after local assessment to detect movement during the check, and label successful output assessment-only and NOT_AUTHORIZED.
• Exercise initial and incremental candidates against real Git fixtures with an offline forge stub, prove no writes, and perform a read-only check against the real marketplace.
• Document the limits: a snapshot cannot reserve a tag, prove review readiness, or guarantee state remains unchanged after it returns. Publication remains a separate feat(ci): cut semver releases so the marketplace has a version to pin #101 increment.

Size: M. Success is rejection of every stale/conflicting fixture while an unchanged real candidate passes without remote writes.

Part of #101.