feat(provider): add Ollama client implementation in new providers crate

- Introduce `owlen-providers` crate with Cargo.toml and lib entry.
- Expose `OllamaClient` handling HTTP communication, health checks, model listing, and streaming generation.
- Implement request building, endpoint handling, and error mapping.
- Parse Ollama tags response and generation stream lines into core types.
- Add shared module re-exports for easy integration with the provider layer.

crates/owlen-providers/src/lib.rs

@@ -0,0 +1,3 @@
+//! Provider implementations for OWLEN.
+
+pub mod ollama;