P1: Publish machine-readable existing Run API and event schemas

[wms2537]

Parent

• Epic Epic: Implement the kertas.runtime/v1alpha1 ManagedSession contract #31
• Ordered work item 1

Dependency

None. This is the first ManagedSession-contract slice.

Scope

Publish a machine-readable, versioned contract for the existing controlled-alpha Run API and append-only Run events without changing their semantics.

Required behavior

• Add a discoverable schema catalog for the existing Run resource, Run creation command, Run event envelope, and current compatibility mode.
• Identify the current mapping as run-as-session/v1.
• Freeze the current public payloads with compatibility fixtures derived through HTTP, not database reads.
• Keep Kertas Project, Outcome, Release, and connector models outside this repository.
• Reject unsupported schema/contract versions explicitly.

Public seams under test

• authenticated contract discovery over HTTP
• existing /v1/runs and /v1/runs/:id/events responses validated against published schemas
• checked static fixtures representing the supported current contract

Acceptance criteria

• schemas are machine-readable, versioned, and included in the OCI image
• a fresh client can discover the supported existing Run contract
• compatibility fixtures validate current API responses and reject incompatible payloads
• no test imports runtime stores or migration modules to validate the public contract
• docs distinguish execution completion from Kertas outcome/release completion
• exact-commit CI, independent review, and post-merge main evidence pass

[wms2537]

Closure evidence:

• reviewed implementation: PR Publish existing Run API contract #54
• exact merge commit: 634f45eddf02f696a1bde2f9a89d33f263aec6bd
• retained post-merge main gate: https://github.com/Straits-AI/managed-agents-runtime/actions/runs/29898030244
• result: passed