config: silence migration warning for legacy enabled=true (no-op)

The legacy-enabled migration shim correctly stripped 'enabled: true' from
dreamer/sidekick config in-memory (the key is no longer valid and the value
is a no-op alias for the new default 'disable: false'), but it surfaced a
config warning suggesting the user had something to fix.

This made users with auto-generated configs (LLM-authored, dashboard-written,
or carried forward from before unification) see a 'Magic Context Config
Warning' dialog on every restart even though their config was semantically
equivalent to the new default. The warning was misleading because nothing
needs migrating — the field is simply obsolete.

Keep the warning only when a real semantic value is present:
  - dreamer.enabled=false → migrated to disable=true (loud, behavior changes)
  - sidekick.enabled=false → migrated to disable=true (loud)
  - historian.enabled=<any> → removed (loud, was always invalid)
  - dreamer.enabled=true → stripped silently (no-op alias)
  - sidekick.enabled=true → stripped silently (no-op alias)

Doctor still cleans both true and false from disk on next run.