fix: Upgrade model to llama3.1:8b-instruct + fix git tool cwd

Change 1: Model Upgrade (Primary Fix)
- Changed default model from llama3.2 to llama3.1:8b-instruct
- llama3.1:8b-instruct is fine-tuned for reliable tool/function calling
- llama3.2 (3B) consistently hallucinated tool output in testing
- Added fallback to qwen2.5:14b if primary unavailable

Change 2: Structured Output Foundation
- Enhanced session init to load real data on first message
- Preparation for JSON schema enforcement

Change 3: Git Tool Working Directory Fix
- Rewrote git_tools.py to use subprocess with cwd=REPO_ROOT
- REPO_ROOT auto-detected at module load time
- All git commands now run from correct directory

Change 4: Session Init with Git Log
- _session_init() reads git log --oneline -15 on first message
- Recent commits prepended to system prompt
- Timmy can now answer 'what's new?' from actual commit data

Change 5: Documentation
- Updated README with new model requirement
- Added CHANGELOG_2025-02-27.md

User must run: ollama pull llama3.1:8b-instruct

All 18 git tool tests pass.

README.md

@@ -59,7 +59,12 @@ make install
 
 # 3. Start Ollama (separate terminal)
 ollama serve
-ollama pull llama3.2
+ollama pull llama3.1:8b-instruct  # Required for reliable tool calling
+
+# Note: llama3.1:8b-instruct is used instead of llama3.2 because it is
+# specifically fine-tuned for reliable tool/function calling.
+# llama3.2 (3B) was found to hallucinate tool output consistently in testing.
+# Fallback: qwen2.5:14b if llama3.1:8b-instruct is not available.
 
 # 4. Launch dashboard
 make dev
@@ -193,7 +198,7 @@ cp .env.example .env
 | Variable | Default | Purpose |
 |----------|---------|---------|
 | `OLLAMA_URL` | `http://localhost:11434` | Ollama host |
-| `OLLAMA_MODEL` | `llama3.2` | Model served by Ollama |
+| `OLLAMA_MODEL` | `llama3.1:8b-instruct` | Model for tool calling. Use llama3.1:8b-instruct for reliable tool use; fallback to qwen2.5:14b |
 | `DEBUG` | `false` | Enable `/docs` and `/redoc` |
 | `TIMMY_MODEL_BACKEND` | `ollama` | `ollama` \| `airllm` \| `auto` |
 | `AIRLLM_MODEL_SIZE` | `70b` | `8b` \| `70b` \| `405b` |