fix(memory): guard provider embedding dimensions to 1536 schema (#563)

- Reject incompatible explicit provider embedding dimensions on create/update
- Ignore previously saved incompatible dimensions at runtime, fall back to 1536
- Share RequiredMemoryEmbeddingDimensions constant from store package
- Remove dimensions input from provider UI (always 1536 per pgvector schema)
- Add HTTP, runtime, and validation unit tests

Closes #548
Supersedes #410

internal/bootstrap/seed_store_test.go

@@ -57,6 +57,9 @@ func (s *seedStubStore) ListUserContextFilesByName(_ context.Context, _ uuid.UUI
 func (s *seedStubStore) DeleteUserContextFile(_ context.Context, _ uuid.UUID, _, _ string) error {
 	return nil
 }
+func (s *seedStubStore) MigrateUserDataOnMerge(_ context.Context, _ []string, _ string) error {
+	return nil
+}
 
 // Remaining interface methods — not exercised.
 func (s *seedStubStore) Create(_ context.Context, _ *store.AgentData) error { return nil }
@@ -111,10 +114,6 @@ func (s *seedStubStore) EnsureUserProfile(_ context.Context, _ uuid.UUID, _ stri
 func (s *seedStubStore) PropagateContextFile(_ context.Context, _ uuid.UUID, _ string) (int, error) {
 	return 0, nil
 }
-func (s *seedStubStore) MigrateUserDataOnMerge(_ context.Context, _ []string, _ string) error {
-	return nil
-}
-
 // ---- Tests ----
 
 // TestSeedUserFiles_PredefinedAgent_UsesAgentLevelUserMD is the primary regression test.