Save edited images as PNG by default to prevent JPEG compression buildup

When no explicit output path is given and source is JPEG/WebP, output
defaults to PNG (lossless). This avoids cumulative quality loss on
repeated edits (brightness → contrast → filter → ...).

Exception: 'convert' operation keeps the original format so explicit
format-change requests still work. Explicit output path always honored.

Also fix quality default mismatch: TS params was 85, Python was 92.
Now both are 92 for when JPEG/WebP output is explicitly requested.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/tools/image.ts

@@ -692,11 +692,15 @@ export const imageEditTool = {
     if (!fs.existsSync(resolved)) return { success: false, error: `File not found: ${resolved}` };
 
     // determine output path
+    // Default to PNG for lossy source formats to avoid cumulative compression loss.
+    // Only exception: 'convert' operation (user explicitly wants a format change).
     let outPath = String(args?.output || '').trim();
     if (!outPath) {
+      const origExt = path.extname(resolved).toLowerCase();
+      const lossyFmts = new Set(['.jpg', '.jpeg', '.webp']);
       const ext = args?.format
         ? `.${args.format}`
-        : (operation === 'convert' ? '.png' : path.extname(resolved));
+        : (operation === 'convert' ? origExt : (lossyFmts.has(origExt) ? '.png' : origExt));
       const base = path.basename(resolved, path.extname(resolved));
       outPath = path.join(path.dirname(resolved), `${base}_${operation}_${Date.now()}${ext}`);
     } else if (!path.isAbsolute(outPath)) {
@@ -718,7 +722,7 @@ export const imageEditTool = {
       text: args?.text ?? '',
       position: args?.position ?? 'bottom-right',
       opacity: args?.opacity ?? 0.5,
-      quality: args?.quality ?? 85,
+      quality: args?.quality ?? 92,
       style: args?.style ?? 'painting',
     };