Extract workflow step executor

[chubes4]

Summary

• Add an internal workflow run context and step executor to share deterministic step resolution, dispatch, normalization, and context updates.
• Route top-level workflow steps and foreach nested steps through the shared executor.
• Route agent steps through the shared ability dispatcher path and preserve agents/chat missing handling.
• Extend workflow runner smoke coverage for agent step error propagation alongside existing top-level and foreach ability coverage.

Testing

• php tests/workflow-runner-smoke.php
• php -l src/Workflows/class-wp-agent-workflow-run-context.php && php -l src/Workflows/class-wp-agent-workflow-step-executor.php && php -l src/Workflows/class-wp-agent-workflow-runner.php && php -l tests/workflow-runner-smoke.php && php -l agents-api.php
• composer phpstan

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the internal workflow step executor refactor and smoke test updates; Chris to review and validate before merge.