feat: add OLLAMA_NUM_CTX config to cap context window (#83)

- Add ollama_num_ctx setting (default 4096) to config.py
- Pass num_ctx option to Ollama in agent.py and agents/base.py
- Add OLLAMA_NUM_CTX to .env.example with usage docs
- Add context_window note in providers.yaml
- Fix mock_settings in test_agent.py for new attribute
- qwen3:30b with 4096 ctx uses ~19GB vs 45GB default

tests/timmy/test_agent.py

@@ -67,6 +67,7 @@ def test_create_timmy_respects_custom_ollama_url():
     ):
         mock_settings.ollama_model = "llama3.2"
         mock_settings.ollama_url = custom_url
+        mock_settings.ollama_num_ctx = 4096
         mock_settings.timmy_model_backend = "ollama"
         mock_settings.airllm_model_size = "70b"
 
@@ -237,6 +238,7 @@ def test_create_timmy_includes_tools_for_large_model():
     ):
         mock_settings.ollama_model = "llama3.1"
         mock_settings.ollama_url = "http://localhost:11434"
+        mock_settings.ollama_num_ctx = 4096
         mock_settings.timmy_model_backend = "ollama"
         mock_settings.airllm_model_size = "70b"
         mock_settings.telemetry_enabled = False