Provider Abstraction

Create Omni/Agent/Provider.hs module to abstract LLM backends.

Current State

• Omni/Agent/Engine.hs contains LLM type with hardcoded OpenRouter logic (lines 267-301)
• chat and chatWithUsage functions make HTTP calls to OpenRouter API (lines 560-594)
• Worker.hs reads OPENROUTER_API_KEY from environment (line 243)

Requirements

1. Create Provider.hs with unified interface:

data Provider
  = OpenRouter { orApiKey :: Text, orModel :: Text }
  | Ollama { olBaseUrl :: Text, olModel :: Text }  
  | AmpCLI { ampPromptFile :: FilePath }

chat :: Provider -> [Tool] -> [Message] -> IO (Either Text ChatResult)

2. Extract OpenRouter implementation:

• Move HTTP logic from Engine.hs chatWithUsage to Provider/OpenRouter.hs
• Keep existing headers (HTTP-Referer, X-Title)
• Parse cost from OpenRouter response usage.cost field

3. Implement Ollama provider:

• Call local Ollama API at http://localhost:11434/api/chat
• Use same message format (OpenAI-compatible)
• Handle tool calls if model supports them
• No API key required

4. Implement Amp subprocess provider:

• Write prompt to temp file
• Spawn 'amp --prompt-file <path>' subprocess
• Capture stdout as response
• Parse any tool outputs from Amp's format

5. Update Engine.hs:

• Replace LLM type usage with Provider
• EngineConfig.engineLLM becomes EngineConfig.engineProvider
• runAgent uses Provider.chat instead of internal chat

6. Update Jr.hs CLI:

• Add --engine flag to 'jr work' command: native|ollama|amp (default: native)
• Pass selected provider to Worker.start

7. Environment variables:

• OPENROUTER_API_KEY for OpenRouter provider
• OLLAMA_HOST for custom Ollama URL (optional, default localhost:11434)
• OLLAMA_MODEL for default model (optional)

Files to modify:

• Omni/Agent/Engine.hs - refactor to use Provider
• Omni/Agent/Worker.hs - accept Provider parameter
• Omni/Jr.hs - add --engine CLI flag

Files to create:

• Omni/Agent/Provider.hs - main module with Provider type
• Omni/Agent/Provider/OpenRouter.hs - extracted OpenRouter logic
• Omni/Agent/Provider/Ollama.hs - new Ollama implementation
• Omni/Agent/Provider/Amp.hs - new Amp subprocess implementation

Testing:

• Run: bild --test Omni/Agent/Provider.hs
• Test OpenRouter with existing API key
• Test Ollama if local instance available
• Amp can be stubbed for now