fix(copilot): address review round 1 — deduplicate prompts, relocate tests

- Slim down the duplicate error-pattern list in _SHARED_TOOL_NOTES
  (prompting.py) to a concise summary that references the guide for details,
  reducing maintenance surface from 5+ near-identical copies to one.
- Move dry_run_loop_test.py from backend/copilot/ (production package) to
  test/copilot/ to match the project's test directory convention.
- Route supplement tests through the public get_sdk_supplement() API instead
  of importing the private _SHARED_TOOL_NOTES symbol.
- Loosen overly-brittle assertions (exact step numbers, exact spacing around
  '/' in error pattern names) while preserving intent as prompt regression
  tests.  Add module-level docstring documenting the deliberate brittleness.