fix: remove unnecessary lock mechanism and upgrade Claude 3 to Claude 4

- Remove sync.RWMutex and rrCounters from Config struct
- Simplify GetModelConfig to use global atomic counter for load balancing
- Remove unnecessary locks from HasProvidersConfig, SaveConfig, etc.
- Add buildModelWithProtocol helper to handle models with existing prefix
- Fix TestCreateProviderReturnsHTTPProviderForOpenRouter to use model_list
- Upgrade all Claude 3 references to Claude 4 across documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/migration/model-list-migration.md

@@ -58,8 +58,8 @@ The new `model_list` configuration offers several advantages:
       "api_base": "https://api.openai.com/v1"
     },
     {
-      "model_name": "claude3",
-      "model": "anthropic/claude-3-sonnet",
+      "model_name": "claude-sonnet-4",
+      "model": "anthropic/claude-sonnet-4",
       "api_key": "sk-ant-your-key"
     },
     {
@@ -83,12 +83,12 @@ The `model` field uses a protocol prefix format: `[protocol/]model-identifier`
 | Prefix | Description | Example |
 |--------|-------------|---------|
 | `openai/` | OpenAI API (default) | `openai/gpt-5.2` |
-| `anthropic/` | Anthropic API | `anthropic/claude-3-opus` |
+| `anthropic/` | Anthropic API | `anthropic/claude-opus-4` |
 | `antigravity/` | Google via Antigravity OAuth | `antigravity/gemini-2.0-flash` |
-| `claude-cli/` | Claude CLI (local) | `claude-cli/claude-3-sonnet` |
+| `claude-cli/` | Claude CLI (local) | `claude-cli/claude-sonnet-4` |
 | `codex-cli/` | Codex CLI (local) | `codex-cli/codex-4` |
 | `github-copilot/` | GitHub Copilot | `github-copilot/gpt-4o` |
-| `openrouter/` | OpenRouter | `openrouter/anthropic/claude-3` |
+| `openrouter/` | OpenRouter | `openrouter/anthropic/claude-sonnet-4` |
 | `groq/` | Groq API | `groq/llama-3.1-70b` |
 | `deepseek/` | DeepSeek API | `deepseek/deepseek-chat` |
 | `cerebras/` | Cerebras API | `cerebras/llama-3.3-70b` |