feat(gui): add cross-origin detection in StdCmdSaveAs (#17)

- Detect document's origin vs current (target) origin
- Route to appropriate workflow:
  - Same origin: standard SaveAs
  - Local → PLM: migration (handled by PLM origin)
  - PLM → Local: export (handled by local origin)
  - PLM → PLM: transfer (handled by target PLM origin)

This provides the infrastructure for Issue #17: Mixed origin workflows.
The actual migration/export dialogs will be implemented in the
respective origin adapters (SiloOrigin, LocalFileOrigin).

src/Gui/CommandDoc.cpp

@@ -768,13 +768,40 @@ void StdCmdSaveAs::activated(int iMsg)
         return;
     }
 
-    // SaveAs uses current origin (allows saving to different origin)
-    FileOrigin* origin = OriginManager::instance()->currentOrigin();
-    if (!origin) {
+    auto* mgr = OriginManager::instance();
+    FileOrigin* currentOrigin = mgr->currentOrigin();
+    FileOrigin* docOrigin = mgr->originForDocument(doc);
+
+    if (!currentOrigin) {
         return;
     }
 
-    origin->saveDocumentAsInteractive(doc);
+    // Determine workflow based on document and target origins
+    OriginType currentType = currentOrigin->type();
+    OriginType docType = docOrigin ? docOrigin->type() : OriginType::Local;
+
+    if (docOrigin == currentOrigin || !docOrigin) {
+        // Same origin or new document - standard SaveAs
+        currentOrigin->saveDocumentAsInteractive(doc);
+    }
+    else if (currentType == OriginType::PLM && docType == OriginType::Local) {
+        // Local → PLM: Migration workflow
+        // The PLM origin's saveDocumentAsInteractive should handle this
+        currentOrigin->saveDocumentAsInteractive(doc);
+    }
+    else if (currentType == OriginType::Local && docType == OriginType::PLM) {
+        // PLM → Local: Export workflow
+        // Use local origin to save without PLM tracking
+        currentOrigin->saveDocumentAsInteractive(doc);
+    }
+    else if (currentType == OriginType::PLM && docType == OriginType::PLM) {
+        // PLM → Different PLM: Transfer workflow
+        currentOrigin->saveDocumentAsInteractive(doc);
+    }
+    else {
+        // Default: use current origin
+        currentOrigin->saveDocumentAsInteractive(doc);
+    }
 }
 
 bool StdCmdSaveAs::isActive()