Make design lifecycle labels deterministic

[chubes4]

Summary

• Removes label mutation tools from the design agent prompt/tool surface so the AI only creates the design-direction issue.
• Adds a deterministic system_task step to transition source issues from status:idea-ready to status:design-ready.
• Updates site-generation plan tests to assert label changes are not AI-driven.

Dependency

• Depends on Add deterministic GitHub issue label task Extra-Chill/data-machine-code#594 for the github_update_issue_labels system task.

Tests

• node tests/scripts/test-homeboy-site-generation-plan.mjs
• node tests/scripts/test-site-generation-loop-workflow.mjs
• node tests/scripts/test-site-generation-loop-proof-assertions.mjs
• for test in tests/scripts/*.mjs; do node "$test" || exit 1; done

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the flow/prompt updates and smoke coverage for deterministic lifecycle labels; Chris directed the determinism approach and remains responsible for review.