Export signed provenance from Every Code runs #443

Goal

Expose parent #441's signed provenance assertion from real Every Code runs and provide enrollment/verification operator surfaces.

Scope

• Add code exec challenge input and assertion output bound to the completed run.
• Collect observed session/thread/turn, model/provider, repository/head, task, role, and transcript digest from runtime-owned state.
• Add an additive JSONL run-provenance event.
• Add key show/rotate and local verification commands for enrollment and interoperability testing.
• Fail closed for missing model, missing Git identity, ephemeral runs, target mismatch, or unavailable transcript evidence.
• Document persistent key storage and container/runtime mounting requirements.

Acceptance Criteria

• A real exec run consumes a Launchplane challenge and emits one verifiable compact assertion.
• Caller text cannot override runtime model family, role, target, or run identity.
• JSONL output is additive and contains only the signed token, not decoded private evidence.
• Key rotation preserves explicit kid trust and does not expose private material.
• Integration tests cover successful export and all fail-closed prerequisites.
• ./build-fast.sh passes with no warnings.

Finish Line

Launchplane workers can obtain signed provenance for the exact Every Code implementer or reviewer run they executed.

Next Action

Begin after the signed provenance crate and golden vector issue is complete.