Add LLM_HEADERS support for custom request headers

Reads LLM_HEADERS as a JSON object from .env and merges it into every
LLM request alongside the existing Authorization header. Useful for
endpoints that require non-standard headers (e.g. x-openclaw-agent-id).
LLM_API_KEY continues to be sent without the "Bearer" prefix in .env.

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

server.js

@@ -922,6 +922,10 @@ app.post('/api/llm/chat', async (req, res) => {
 
   const headers = {};
   if (apiKey) headers['Authorization'] = `Bearer ${apiKey}`;
+  try {
+    const extra = process.env.LLM_HEADERS ? JSON.parse(process.env.LLM_HEADERS) : {};
+    Object.assign(headers, extra);
+  } catch { console.warn('[llm] LLM_HEADERS is not valid JSON — ignored'); }
 
   try {
     const result = await httpPost(url, headers, { model: mdl, messages: allMessages, stream: false });