feat(import): remember the user's import selection

Yaak now remembers, per item of a linked import source, whether the user
wants it. A mapping row that still points at a model means wanted, so
re-imports merge into it; a row with no model behind it means not wanted,
so re-imports leave it alone. Wanted-ness is structural rather than a flag:
deleting the model locally, or turning the item down in the preview, is
what makes it not wanted.

- import_source_resources drops `snapshot` for a nullable `content_hash`
  and lets `model_id` be NULL. The migration keeps every beta row's
  key to model mapping and starts hashes empty; until a hash is recorded,
  a difference can't be attributed to either side, so the item is offered
  once as a conflict that keeps local changes.
- comparable() ignores sortPriority. Importers number it from source
  order, so inserting one operation used to plan a fake update for
  everything after it. The trade is that pure reorders don't propagate.
- Keys the user turned down come back as unchecked "not imported" rows
  instead of being re-offered as new resources or resurrected. Checking
  one imports it under the same source key and pulls in the folders it
  needs.
- An imported resource the source moves into a folder that isn't imported
  is offered as a deletion, with a tooltip pointing at the folder.
- Plan-time source resolution binds by source-key overlap instead of by
  path, so a renamed or moved file merges into what it created and heals
  the stored origin. Several sources sharing keys never guess-merge: the
  plan says so and imports everything as new.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates-cli/yaak-cli/src/commands/import_export.rs

@@ -113,6 +113,10 @@ fn format_skipped(items: &[ImportPlanItem]) -> Option<String> {
     if keep_local > 0 {
         parts.push(format!("{keep_local} with local edits"));
     }
+    let not_imported = count(ImportPlanAction::NotImported);
+    if not_imported > 0 {
+        parts.push(format!("{not_imported} previously not imported"));
+    }
     let unchanged = count(ImportPlanAction::Unchanged);
     if unchanged > 0 {
         parts.push(format!("{unchanged} unchanged"));