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/i18n/messages/en.settings.ts

@@ -337,6 +337,14 @@ export const settingsDict = {
   'settings.agents.page.permissions.addCustomRule': 'Add Custom Rule',
   'settings.agents.page.permissions.permissionPlaceholder': 'Permission...',
   'settings.agents.page.permissions.patternPlaceholder': 'Pattern (e.g. *)',
+  'settings.page.behavior.title': 'Behavior',
+  'settings.behavior.page.title': 'Behavior',
+  'settings.behavior.page.warning.title': 'Global rules are combined with project rules',
+  'settings.behavior.page.warning.description': 'Changes made here update {path}. OpenCode also includes any project-level AGENTS.md rules when they exist.',
+  'settings.behavior.page.section.systemPrompt': 'Global AGENTS.md',
+  'settings.behavior.page.field.systemPromptPlaceholder': 'You are a helpful AI assistant...\n\nUse this space to define absolute rules for how the AI should behave across all sessions and providers.',
+  'settings.behavior.page.toast.saved': 'Behavior saved successfully',
+  'settings.behavior.page.toast.saveFailed': 'Failed to save behavior',
   'settings.commands.sidebar.title': 'Commands',
   'settings.commands.sidebar.total': 'Total {count}',
   'settings.commands.sidebar.empty.title': 'No commands configured',