feat: add Tinfoil private inference provider (#62)

* feat: add Tinfoil private inference provider

Add a dedicated Tinfoil LLM backend (`LLM_BACKEND=tinfoil`) for
Tinfoil's private inference service (https://tinfoil.sh).

The existing `openai_compatible` backend cannot be used with Tinfoil
because rig-core 0.30.0 defaults to the OpenAI Responses API
(`/v1/responses`), which Tinfoil does not support — it only implements
the Chat Completions API (`/v1/chat/completions`), returning 403
"shim: path not allowed" when hit on the responses endpoint.

Rather than changing `openai_compatible` to use Chat Completions (which
would break users expecting the Responses API), this adds a dedicated
provider that explicitly uses rig's `.completions_api()` client.

This also lays the groundwork for integrating Tinfoil's privacy wrapper
client (enclave attestation, TLS certificate pinning) once their Rust
SDK is available. The provider implementation can be swapped to use the
Tinfoil Rust client without changing the LlmProvider interface.

Configuration:
  LLM_BACKEND=tinfoil
  TINFOIL_API_KEY=tk_...
  TINFOIL_MODEL=kimi-k2-5   # optional, default

* style: fix rustfmt formatting in Tinfoil provider

* style: remove unnecessary tin_foil alias for Tinfoil backend

* Update src/llm/mod.rs

Co-authored-by: Copilot <[email protected]>

* fix: add tinfoil field to LlmConfig test fixture

* style: fix rustfmt output in session manager

---------

Co-authored-by: firat.sertgoz <[email protected]>
Co-authored-by: Copilot <[email protected]>
Co-authored-by: Illia Polosukhin <[email protected]>

src/setup/wizard.rs

@@ -1034,6 +1034,7 @@ impl SetupWizard {
             anthropic: None,
             ollama: None,
             openai_compatible: None,
+            tinfoil: None,
         };
 
         match create_llm_provider(&config, session) {