release: publish the stable release-train contract and AI harness in Base 1.11.0 #2499

Purpose

Track the public, user-facing release-train contract deferred from the Base 1.10.1 process-hardening patch. This work belongs in the next minor release because it may add or stabilize supported basectl behavior and the AI-assisted release harness.

Scope

• Define the stable basectl release train command surface, exit codes, and JSON contract.
• Add compatibility tests for the documented release-train commands and machine-readable output.
• Make the Codex release skill consume the stable command contract rather than duplicating release policy.
• Add replayable AI-harness fixtures for milestone, issue, PR, hosted-check, BOM, and public-artifact readback.
• Document supported automation boundaries, approval points, and recovery behavior.
• Validate the contract against Base and the companion repositories before publishing 1.11.0.

Release gate

No open issue assigned to v1.11.0, unresolved required review, failing required check, or unverified public artifact may remain when the release is cut. Component-specific remediation stays in the component repository and milestone that owns it.