feat: add ollama support (#536)

.changeset/free-bikes-smile.md

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+---
+'task-master-ai': minor
+---
+
+Add Ollama as a supported AI provider.
+
+- You can now add it by running `task-master models --setup` and selecting it.
+- Ollama is a local model provider, so no API key is required.
+- Ollama models are available at `http://localhost:11434/api` by default.
+- You can change the default URL by setting the `OLLAMA_BASE_URL` environment variable or by adding a `baseUrl` property to the `ollama` model role in `.taskmasterconfig`.
+  - If you want to use a custom API key, you can set it in the `OLLAMA_API_KEY` environment variable.