Remove the unsafe `# type: ignore` suppression on line 111 of
`legacy_migrator.py` and replace it with an explicit
`assert existing_plan.id is not None` statement. This provides
proper type narrowing to the type checker while preserving the
logical correctness guaranteed by the preceding `if existing_plan:`
guard.
Also adds a new BDD scenario in `legacy_migrator_coverage.feature`
that explicitly exercises the code path where an existing plan with
a non-None id is found during migration, verifying the assert-based
type narrowing works correctly end-to-end.
ISSUES CLOSED: #3051
HAL9000
freemo