Fix: v3.6.0 migration writes _messages to course, not config

[Copilot]

Fixes #369

Fix

• Retargeted the to v3.6.0 migration from getSpoorConfig() (config doc) to a new getSpoorCourse() helper (course doc), so _messages: {} is written to the correct document

Update

• Test fixtures for the to v3.6.0 block updated from { _type: 'config', _spoor: ... } to { _type: 'course', _spoor: ... } to match the corrected target

Testing

1. Verify a legacy course bundle (spoor <3.6.0) without _messages on the course migrates successfully and the course doc gains _spoor._messages: {}
2. Verify a course already containing _spoor._messages stops (no-op) as expected
3. Verify a course without _spoor stops as expected