Keep contribution preparation owner-initiated

[hamzamerzic]

Summary

• make private preparation begin only from an explicit owner request or Contribute project handoff
• let Contribute Projects own discovery instead of prompting at the end of unrelated work
• keep refinement feedback non-authorizing and require a fresh preparation request after changes
• preserve the separate, explicit approval boundary for every public action
• release Contribute 0.5.19

Context

This replaces the unsolicited end-of-task question-card pattern introduced in #17. It also addresses the refinement ambiguity explored in the closed #31, but routes renewed intent through Contribute Projects instead of automatically re-offering the same contribution in chat.

Validation

• npm test (142 Node tests, 17 Python tests)
• git diff --check