feat(files): add save-as-new vs overwrite choice for library file edits (#564)

Editing a file from the library used to silently supersede it: the worker auto-saved every result as a new version and the leaf-only listing hid the original, which read as a destructive overwrite. Tool pages now show a per-edit choice for library-sourced files. The default saves the result as an independent new file and keeps the original; picking overwrite keeps the old superseding-version behavior.

The client sends a saveMode multipart field next to fileId, validated with a 400 on unknown values, and autoSaveToLibrary branches on it. Every hand-written route that honors fileId parses the field the same way as the factory. The review panel shows where an auto-saved result went instead of offering a second, duplicate save. Tools whose route or submitter ignores fileId keep the selector hidden via a shared unsupported-tools set, and the choice resets to the non-destructive default whenever a new file is staged.

Closes #495

apps/web/src/components/files/file-details.tsx

@@ -432,7 +432,9 @@ export function FileDetails({ mobile = false }: FileDetailsProps) {
       navigate("/", { state: { fromLibrary: true } });
     }
 
-    // Set serverFileId on each entry so tool processing creates new versions
+    // Set serverFileId on each entry so tool processing auto-saves the result
+    // to the library: an independent new file by default, or a superseding
+    // version when the user picks overwrite (#495)
     setTimeout(() => {
       const store = useFileStore.getState();
       for (let i = 0; i < valid.length; i++) {