Add sandbox completion outcome artifacts

[chubes4]

Summary

• Add a generic wp-codebox/sandbox-completion-outcome/v1 artifact at files/completion-outcome.json for sandbox agent recipe runs.
• Surface the completion outcome through recipe-run --json, wp-codebox/run-agent-task, batch run outputs, and session artifact refs.
• Document the generic completion contract and cover the WordPress ability/runner schema in smoke tests.

Fixes #430.

Tests

• git diff --check
• npm run build
• npm run recipe-runtime-evidence-smoke
• npm run wordpress-plugin-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implementing the generic completion outcome contract, tests, docs, and PR preparation for Chris to review.