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/ui/src/lib/settings/metadata.ts

@@ -7,6 +7,7 @@ export type SettingsPageSlug =
   | 'providers'
   | 'usage'
   | 'agents'
+  | 'behavior'
   | 'commands'
   | 'mcp'
   | 'skills.installed'
@@ -103,6 +104,13 @@ export const SETTINGS_PAGE_METADATA: readonly SettingsPageMeta[] = [
     kind: 'split',
     keywords: ['agent', 'agents', 'prompts', 'tools', 'permissions'],
   },
+  {
+    slug: 'behavior',
+    title: 'Behavior',
+    group: 'opencode',
+    kind: 'single',
+    keywords: ['behavior', 'agents.md', 'system prompt', 'global rules', 'instructions', 'override'],
+  },
   {
     slug: 'commands',
     title: 'Commands',