Story 12: Update story and specs for Claude integration

Story Updates:
- Unified model dropdown with section headers (Anthropic, Ollama)
- Auto-detect provider from model name (claude-* prefix)
- API key prompt on first Claude model use
- Secure storage in OS keychain via keyring crate
- 200k token context window for Claude models

Spec Updates (AI_INTEGRATION.md):
- Document Anthropic provider implementation
- Anthropic API protocol (SSE streaming, tool format)
- Tool format conversion between internal and Anthropic formats
- API key storage in OS keychain
- Unified dropdown UI flow

Spec Updates (STACK.md):
- Add keyring crate for secure API key storage
- Add eventsource-stream for Anthropic SSE streaming
- Document automatic provider detection
- Update API key management approach

.living_spec/specs/tech/STACK.md

@@ -31,12 +31,18 @@ To support both Remote and Local models, the system implements a `ModelProvider`
     *   Abstract the differences between API formats (OpenAI-compatible vs Anthropic vs Gemini).
     *   Normalize "Tool Use" definitions, as each provider handles function calling schemas differently.
 *   **Supported Providers:**
-    *   **Anthropic:** Focus on Claude 3.5 Sonnet for coding tasks.
-    *   **Google:** Gemini 1.5 Pro for massive context windows.
     *   **Ollama:** Local inference (e.g., Llama 3, DeepSeek Coder) for privacy and offline usage.
-*   **Configuration:**
-    *   Provider selection is runtime-configurable by the user.
-    *   API Keys must be stored securely (using OS native keychain where possible).
+    *   **Anthropic:** Claude 3.5 models (Sonnet, Haiku) via API for coding tasks (Story 12).
+*   **Provider Selection:**
+    *   Automatic detection based on model name prefix:
+        *   `claude-` → Anthropic API
+        *   Otherwise → Ollama
+    *   Single unified model dropdown with section headers ("Anthropic", "Ollama")
+*   **API Key Management:**
+    *   Anthropic API key stored in OS keychain (macOS Keychain, Windows Credential Manager, Linux Secret Service)
+    *   Uses `keyring` crate for cross-platform secure storage
+    *   On first use of Claude model, user prompted to enter API key
+    *   Key persists across sessions (no re-entry needed)
 
 ## Tooling Capabilities
 
@@ -90,7 +96,9 @@ To support both Remote and Local models, the system implements a `ModelProvider`
     *   `ignore`: Fast recursive directory iteration respecting gitignore.
     *   `walkdir`: Simple directory traversal.
     *   `tokio`: Async runtime.
-    *   `reqwest`: For LLM API calls (if backend-initiated).
+    *   `reqwest`: For LLM API calls (Anthropic, Ollama).
+    *   `eventsource-stream`: For Server-Sent Events (Anthropic streaming).
+    *   `keyring`: Secure API key storage in OS keychain.
     *   `uuid`: For unique message IDs.
     *   `chrono`: For timestamps.
     *   `tauri-plugin-dialog`: Native system dialogs.