feat(mcp-server): add nexus_vision tool and fix upload MIME detection

- New tool nexus_vision: describe an image via a vision chat model
  (default google/gemini-3.1-flash-lite) and record result in journal.
- NordRouter /media/upload now detects MIME from file extension so
  data: URLs use the correct type (fixes HTTP 415 for images/audio/video).
- HttpClient supports FormData bodies (undici) for future multipart use.

NexusAI/packages/core/src/providers/nordrouter/media.ts

@@ -136,7 +136,21 @@ export class NordRouterMediaProvider implements MediaProvider {
       throw new NexusError('IO', `Upload file not found: ${input.path}`);
     }
     const buf = fs.readFileSync(input.path);
-    const mime = input.mimeType || 'application/octet-stream';
+    const ext = input.path.split('.').pop()?.toLowerCase() || '';
+    const mimeByExt: Record<string, string> = {
+      png: 'image/png',
+      jpg: 'image/jpeg',
+      jpeg: 'image/jpeg',
+      webp: 'image/webp',
+      gif: 'image/gif',
+      mp4: 'video/mp4',
+      mov: 'video/quicktime',
+      wav: 'audio/wav',
+      mp3: 'audio/mpeg',
+      ogg: 'audio/ogg',
+      m4a: 'audio/mp4',
+    };
+    const mime = input.mimeType || mimeByExt[ext] || 'application/octet-stream';
     const name = input.name || input.path.split('/').pop() || 'file';
     const dataUrl = `data:${mime};base64,${buf.toString('base64')}`;
     const res = await this.client.http.requestJson<{ url?: string }>({

NexusAI/packages/core/src/shared/http.ts

@@ -1,4 +1,4 @@
-import { fetch, ProxyAgent, type RequestInit, type Response } from 'undici';
+import { fetch, ProxyAgent, FormData, type RequestInit, type Response } from 'undici';
 import { NexusError, codeFromHttpStatus } from './errors.js';
 import { sleep } from './polling.js';
 
@@ -83,10 +83,15 @@ export class HttpClient {
 
     const headers: Record<string, string> = { ...this.opts.defaultHeaders, ...options.headers };
     let bodyStr: string | undefined;
+    let bodyForm: FormData | undefined;
     if (options.body !== undefined) {
-      bodyStr = typeof options.body === 'string' ? options.body : JSON.stringify(options.body);
-      if (!headers['Content-Type'] && !headers['content-type']) {
-        headers['Content-Type'] = 'application/json';
+      if (options.body instanceof FormData) {
+        bodyForm = options.body;
+      } else {
+        bodyStr = typeof options.body === 'string' ? options.body : JSON.stringify(options.body);
+        if (!headers['Content-Type'] && !headers['content-type']) {
+          headers['Content-Type'] = 'application/json';
+        }
       }
     }
 
@@ -96,7 +101,7 @@ export class HttpClient {
         const init: RequestInit & { dispatcher?: ProxyAgent } = {
           method,
           headers,
-          body: bodyStr,
+          body: bodyForm ?? bodyStr,
           signal: AbortSignal.timeout(timeoutMs),
         };
         if (this.proxyAgent) init.dispatcher = this.proxyAgent;