Migrate to litellm for model compatability

[trevormells]

Summary

This PR migrates ApplyPilot’s LLM layer to a LiteLLM-based adapter and standardizes provider/model configuration across CLI, wizard, docs, and runtime checks. It reduces provider-specific logic, adds Anthropic support, and tightens test coverage around LLM resolution/client behavior.

---

What Changed

• Replaced the custom HTTP-based LLM implementation with a thin LiteLLM wrapper in llm.py.
• Added a single resolve_llm_config() contract for provider/model/api key resolution.
• Expanded provider support:
  • GEMINI_API_KEY
  • OPENAI_API_KEY
  • ANTHROPIC_API_KEY
  • LLM_URL (OpenAI-compatible local endpoint)
  • LLM_API_KEY (generic key fallback)
• Standardized model semantics:
  • Provider-prefixed models supported (e.g. openai/gpt-4o-mini, gemini-3.0-flash)
  • Inference order for provider defaults when LLM_MODEL is not set
• Updated LLM call sites to use client.chat(..., max_output_tokens=...) and removed ask() usage.
• Updated applypilot init flow to allow saving multiple provider credentials and explicit LLM_MODEL.
• Updated doctor/tier checks and failure messaging to match the new config contract.
• Updated docs and .env.example for new provider options and model format.
• Added optional Gemini smoke test + pytest marker config.
• Added unit tests for LLM config resolution and LiteLLM client request behavior.

---

Tests

Added

• test_llm_resolution.py
• test_llm_client.py
• test_gemini_smoke.py (optional smoke: @pytest.mark.smoke)

Suggested commands

pytest -q tests/test_llm_resolution.py tests/test_llm_client.py
GEMINI_API_KEY=... pytest -m smoke -q tests/test_gemini_smoke.py

Notes

• Runtime LLM routing now follows LLM_MODEL provider prefix when multiple providers are configured.
• Local OpenAI-compatible endpoints are supported via LLM_URL (with optional LLM_API_KEY).
• Default model selections were refreshed (Gemini/OpenAI/Anthropic/local).

[trevormells]

addresses a wide range of model compatibility issues that have been surfaced in issues
#19
#4
#9
#16
@Pickle-Pixel