feat: Add OpenCode provider integration with official brand icons

This commit integrates OpenCode as a new AI provider and updates all provider
icons with their official brand colors for better visual recognition.

**OpenCode Provider Integration:**
- Add OpencodeProvider class with CLI-based execution
- Support for OpenCode native models (opencode/) and Bedrock models
- Proper event normalization for OpenCode streaming format
- Correct CLI arguments: --format json (not stream-json)
- Event structure: type, part.text, sessionID fields

**Provider Icons:**
- Add official OpenCode icon (white square frame from opencode.ai)
- Add DeepSeek icon (blue whale #4D6BFE)
- Add Qwen icon (purple gradient #6336E7 → #6F69F7)
- Add Amazon Nova icon (AWS orange #FF9900)
- Add Mistral icon (rainbow gradient gold→red)
- Add Meta icon (blue #1877F2)
- Update existing icons with brand colors:
  * Claude: #d97757 (terra cotta)
  * OpenAI/Codex: #74aa9c (teal-green)
  * Cursor: #5E9EFF (bright blue)

**Settings UI Updates:**
- Update settings navigation to show OpenCode icon
- Update model configuration to use provider-specific icons
- Differentiate between OpenCode free models and Bedrock-hosted models
- All AI models now display their official brand logos

**Model Resolution:**
- Add isOpencodeModel() function to detect OpenCode models
- Support patterns: opencode/, opencode-*, amazon-bedrock/*
- Update getProviderFromModel to recognize opencode provider

Note: Some unit tests in opencode-provider.test.ts need updating to match
the new event structure and CLI argument format.

apps/ui/src/components/views/settings-view/cli-status/opencode-cli-status.tsx

@@ -1,7 +1,8 @@
 import { Button } from '@/components/ui/button';
-import { CheckCircle2, AlertCircle, RefreshCw, XCircle, Bot } from 'lucide-react';
+import { CheckCircle2, AlertCircle, RefreshCw, XCircle } from 'lucide-react';
 import { cn } from '@/lib/utils';
 import type { CliStatus } from '../shared/types';
+import { OpenCodeIcon } from '@/components/ui/provider-icon';
 
 export type OpencodeAuthMethod =
   | 'api_key_env' // ANTHROPIC_API_KEY or other provider env vars
@@ -169,7 +170,7 @@ export function OpencodeCliStatus({
         <div className="flex items-center justify-between mb-2">
           <div className="flex items-center gap-3">
             <div className="w-9 h-9 rounded-xl bg-gradient-to-br from-brand-500/20 to-brand-600/10 flex items-center justify-center border border-brand-500/20">
-              <Bot className="w-5 h-5 text-brand-500" />
+              <OpenCodeIcon className="w-5 h-5 text-brand-500" />
             </div>
             <h2 className="text-lg font-semibold text-foreground tracking-tight">OpenCode CLI</h2>
           </div>