feat(log): add repo/pr correlation fields to the self-host job audit line

[JSONbored]

The self-host job audit line (src/selfhost/audit.ts:14) carries event/job_id/payload_type/latency but no repo or PR number, so grepping docker compose logs for 'everything that happened to PR #123' is impossible — you only get the job id. Extend AuditEvent with optional repo and pr_number fields extracted from the job payload (like extractPayloadType already does at audit.ts:32) and thread them through logAudit callers.

Deliverables

• Add optional repo?:string and pr_number?:number to AuditEvent and include them in the emitted JSON when present
• Add an extractPayloadContext(payload) helper (mirrors extractPayloadType) that safely pulls repo/pr fields, returning undefined on non-object/missing
• Do not log full payloads; only the two correlation fields
• Unit tests for extractPayloadContext: present, missing, malformed-JSON, wrong-types (both nullish arms)

References

• src/selfhost/audit.ts:14
• src/selfhost/audit.ts:26
• src/selfhost/audit.ts:32

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size: S · gittensor:feature

[linear-code]

JSO-2204