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.

libs/model-resolver/src/resolver.ts

@@ -15,6 +15,7 @@ import {
   DEFAULT_MODELS,
   PROVIDER_PREFIXES,
   isCursorModel,
+  isOpencodeModel,
   stripProviderPrefix,
   type PhaseModelEntry,
   type ThinkingLevel,
@@ -68,6 +69,13 @@ export function resolveModelString(
     return modelKey;
   }
 
+  // OpenCode model - pass through unchanged
+  // Supports: opencode/big-pickle, opencode-sonnet, amazon-bedrock/anthropic.claude-*
+  if (isOpencodeModel(modelKey)) {
+    console.log(`[ModelResolver] Using OpenCode model: ${modelKey}`);
+    return modelKey;
+  }
+
   // Full Claude model string - pass through unchanged
   if (modelKey.includes('claude-')) {
     console.log(`[ModelResolver] Using full Claude model string: ${modelKey}`);