feat: agent brain (ollama) and chat ui

.living_spec/stories/archive/03_llm_ollama.md

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+# Story: The Agent Brain (Ollama Integration)
+
+## User Story
+**As a** User
+**I want to** connect the Assistant to a local Ollama instance
+**So that** I can chat with the Agent and have it execute tools without sending data to the cloud.
+
+## Acceptance Criteria
+*   [ ] Backend: Implement `ModelProvider` trait/interface.
+*   [ ] Backend: Implement `OllamaProvider` (POST /api/chat).
+*   [ ] Backend: Implement `chat(message, history, provider_config)` command.
+    *   [ ] Must support passing Tool Definitions to Ollama (if model supports it) or System Prompt instructions.
+    *   [ ] Must parse Tool Calls from the response.
+*   [ ] Frontend: Settings Screen to toggle "Ollama" and set Model Name (default: `llama3`).
+*   [ ] Frontend: Chat Interface.
+    *   [ ] Message History (User/Assistant).
+    *   [ ] Tool Call visualization (e.g., "Running git status...").
+
+## Out of Scope
+*   Remote Providers (Anthropic/OpenAI) - Future Story.
+*   Streaming responses (wait for full completion for MVP).
+*   Complex context window management (just send full history for now).