M3: Code offload job contract + result recording #32

Context

PRD: docs/PRD.md §9, milestone M3. Ecosystem PRD phase M3 (Code offload via Sync jobs).

Goal

Standardize kind=code_offload payload/result so CLI/human creates a job, an Aixgo Code (or human ax:go) worker claims it, and the job records PR URL / status. Optionally notify the session mailbox.

Out of scope

• Running Aixgo Code inside this repo
• Auto-merge
• Full GitHub App installation UX

Contract

Document and validate:

• Create payload: repo, issue/PR context, gate hints, callback session/thread ids as needed
• Statuses: queued → claimed → running → done|failed|cancelled
• Result: PR URL, status summary, error string
• Mailbox: on terminal status, append a system/agent message to the related thread when ids present

Acceptance criteria

• Documented JSON schema (README or docs/) for code_offload payload/result
• Create + claim + result PUT/update path works with CAS
• Tests cover happy path + failed job result
• make check green

Dependencies

Jobs + claim on serve path (#6 + wire-up issue); attach/notify helpful but not strictly required for recording results.

How to hand to Aixgo Code

Apply ax:go after M2 attach/notify or when Code integration is prioritized.