Add workflow result artifacts/logs fields

[chubes4]

Summary

Fixes #403.

Adds first-class artifacts and logs arrays to WP_Agent_Workflow_Run_Result so workflow consumers no longer need to smuggle those values through free-form metadata.

Motivation

Downstream Data Machine workflow recording currently copies artifacts and logs out of metadata because the Agents API workflow result contract has no dedicated fields:

• https://github.com/Extra-Chill/data-machine/blob/main/inc/Core/AgentsApiWorkflowJobRecorder.php
• https://github.com/Extra-Chill/data-machine/blob/main/docs/agents-api-workflow-bridge.md

This PR makes the contract explicit while keeping the persistence model storage-neutral and table-free.

Design

• Adds normalized list fields artifacts and logs to WP_Agent_Workflow_Run_Result.
• Preserves those fields through construction, from_array(), to_array(), with(), runner options, and recorder payloads.
• Maps workflow artifacts into canonical run result envelope artifact_refs.
• Adds first-class logs support to the canonical run result envelope so logs do not get re-buried in metadata at that boundary.
• Updates the agents/run-workflow input/output schemas to document and expose the new fields.

Back compatibility

This is additive only:

• Existing constructor argument order is preserved; new arguments are appended.
• Existing serialized results without artifacts or logs default both fields to empty arrays.
• Existing metadata passthrough remains untouched, including metadata keys named artifacts or logs.
• Existing evidence_refs behavior remains unchanged.

Test evidence

• php tests/workflow-runner-smoke.php passed: 67 passed, 0 failed.
• php tests/agents-workflow-ability-smoke.php passed: 27 passed, 0 failed.
• php tests/run-result-envelope-smoke.php passed: 23 assertions passed.
• composer test passed.
• composer phpstan passed: No errors.

AI assistance

• AI assistance: Yes
• Tool(s): opencode — Claude Fable 5 (Anthropic) orchestrator, GPT 5.5 (OpenAI) implementation subagent
• Used for: Implemented the additive workflow result contract fields, tests, verification, and PR preparation.