feat: add Behavior settings page for global AGENTS.md (#1079)

* feat: add Behavior settings page for global AGENTS.md

- Add new 'Behavior' settings page to manage global system prompt
- Sync global behavior prompt to ~/.config/opencode/AGENTS.md
- Add GET/PUT /api/behavior/agents-md endpoints with 1MB size limit
- Add AbortController guards and parallel fetches in BehaviorPage
- Add i18n translations for en, es, ko, pt-BR, uk, zh-CN
- Add globalBehaviorPrompt to DesktopSettings type and sanitizer
- Add /api/behavior to express.json() body-parser whitelist
- Ensure atomic save: AGENTS.md written before settings updated

* fix: address Greptile review feedback

- Rename misleading 'trimmed' variable to 'value' in settings-helpers.js
- Add Content-Length guard for /api/behavior before 50mb JSON parser
- Use express.json({ limit: '1mb' }) for behavior endpoints
- Add actual translations for es, ko, pt-BR, uk, zh-CN locales

* fix(vscode): support behavior settings endpoint

* fix(behavior): wait for settings persistence

* fix(behavior): end agents file with newline

* fix(behavior): avoid duplicate textarea resize handles

* fix(behavior): clarify global rules copy

* fix(behavior): move rules note into tooltip

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/web/server/lib/opencode/routes.js

@@ -1,4 +1,7 @@
 import { createProjectIdFromPath } from '../projects/project-id.js';
+import fs from 'fs';
+import os from 'os';
+import path from 'path';
 
 export const registerOpenCodeRoutes = (app, dependencies) => {
   const {
@@ -295,4 +298,55 @@ export const registerOpenCodeRoutes = (app, dependencies) => {
       return res.status(500).json({ error: error.message || 'Failed to update working directory' });
     }
   });
+
+  // Behavior / Global AGENTS.md endpoints
+  const AGENTS_MD_PATH = path.join(os.homedir(), '.config', 'opencode', 'AGENTS.md');
+  const MAX_BEHAVIOR_PROMPT_SIZE = 1024 * 1024; // 1 MB
+
+  app.get('/api/behavior/agents-md', async (_req, res) => {
+    try {
+      try {
+        await fs.promises.access(AGENTS_MD_PATH);
+      } catch {
+        return res.json({ content: '', exists: false });
+      }
+      const content = await fs.promises.readFile(AGENTS_MD_PATH, 'utf8');
+      return res.json({ content, exists: true });
+    } catch (error) {
+      console.error('Failed to read AGENTS.md:', error);
+      return res.status(500).json({ error: 'Failed to read AGENTS.md' });
+    }
+  });
+
+  app.put('/api/behavior/agents-md', async (req, res) => {
+    try {
+      const content = typeof req.body?.content === 'string' ? req.body.content : '';
+
+      if (content.length > MAX_BEHAVIOR_PROMPT_SIZE) {
+        return res.status(413).json({ error: `Content exceeds maximum size of ${MAX_BEHAVIOR_PROMPT_SIZE} bytes` });
+      }
+
+      // Ensure parent directory exists
+      const parentDir = path.dirname(AGENTS_MD_PATH);
+      try {
+        await fs.promises.access(parentDir);
+      } catch {
+        await fs.promises.mkdir(parentDir, { recursive: true });
+      }
+
+      await fs.promises.writeFile(AGENTS_MD_PATH, content, 'utf8');
+
+      // Refresh OpenCode so it picks up the new AGENTS.md without a full restart
+      try {
+        await refreshOpenCodeAfterConfigChange('global behavior (AGENTS.md) updated');
+      } catch {
+        // Non-fatal: file was written successfully
+      }
+
+      return res.json({ success: true });
+    } catch (error) {
+      console.error('Failed to write AGENTS.md:', error);
+      return res.status(500).json({ error: error.message || 'Failed to write AGENTS.md' });
+    }
+  });
 };