PM-4652: persist assigned copilot on draft creation

[jmgasper]

What was broken

• When a user selected a copilot before clicking New, the created draft reopened without that copilot saved.

Root cause

• The initial challenge-creation flow reset the form from fetched challenge data before creating the Copilot resource assignment, so the pre-create selection was dropped.

What was changed

• Persist the selected copilot resource immediately after the first draft is created.
• Keep the selected copilot populated through the post-create form reset.
• Update the challenge editor documentation to match the create flow.

Any added/updated tests

• Added a ChallengeEditorForm regression test covering Assign yourself during initial draft creation.
• Reran the full platform-ui test suite, lint, and build.

---