Add DeepSeek provider for ForgeCode agent

[jpleva91]

Context

ShellForge currently supports Anthropic and Ollama providers. DeepSeek offers cheap, fast inference via an OpenAI-compatible API — ideal for high-volume agent work.

Scope

1. Create internal/llm/deepseek.go implementing the Provider interface
2. DeepSeek uses OpenAI-compatible API at https://api.deepseek.com/v1/chat/completions
3. Env vars: DEEPSEEK_API_KEY, DEEPSEEK_MODEL (default: deepseek-chat)
4. Add provider switch case in cmd/shellforge/main.go
5. Add tests

Why Now

ForgeCode issue worker needs cheap providers for high-volume autonomous work. DeepSeek is the cost leader for bulk coding tasks.