Surface agent sandbox transcript summaries

[chubes4]

Summary

• Add a compact agentResult summary to recipe-run output and mirror it from the WordPress runner as agent_result.
• Write additive files/agent-result.json and files/transcript.json artifacts for agent sandbox workflow steps.
• Preserve existing artifact evidence while documenting no-op/actionability semantics for downstream orchestrators.

Closes #172.

Testing

• npm run build
• npm run agent-sandbox-code-smoke
• npm run wordpress-plugin-smoke
• git diff --check
• npm run check was started and passed through the focused and early full-suite smokes, then was aborted by controller during recipe-staged-files-smoke to finish the PR.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the additive agent sandbox summary/artifact contract, updated smoke coverage/docs, and ran verification commands. Chris remains responsible for review and merge.