feat: deepseek llm provider

[virinchiv]

Description

Adds DeepSeek as a built-in LLM provider (pkg/llm/deepseek) implementing interfaces.LLMClient, alongside the existing OpenAI, Anthropic, and Gemini clients.
DeepSeek exposes an OpenAI-compatible Chat Completions API, so the client uses the openai-go SDK purely as transport (no new dependency) but implements the interface independently.

• Defaults the base URL to https://api.deepseek.com (overridable via llm.WithBaseURL).
• Sends system prompts with the system role (DeepSeek rejects OpenAI's developer role).
• Maps deepseek-reasoner's reasoning_content to LLMStreamChunk.ThinkingDelta (streaming) and LLMResponse.Metadata["reasoning_content"] (non-streaming).
• Maps ResponseFormatJSON to json_object (DeepSeek doesn't support OpenAI's json_schema type; any Schema is ignored).

Registers the LLMProviderDeepSeek constant and wires the provider into the cmd and examples config factories. Supports deepseek-chat and deepseek-reasoner models.

Type of change

New feature (non-breaking change which adds functionality)

Related issues

Fixes #37

Checklist

• [ Y ] I have run make check
• [ Y ] I have run task examples:all
• [ Y ] I have run make tidy if I added or removed dependencies
• [ Y ] Commit messages follow conventional commits (e.g. feat:, fix:, docs:)
• [ Y ] I have added/updated tests for my changes
• [ Y ] Documentation is updated if needed

[codecov]

Codecov Report

❌ Patch coverage is 67.00508% with 65 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/llm/deepseek/client.go	67.00%	61 Missing and 4 partials ⚠️

📢 Thoughts on this report? Let us know!

[vinodvx]

Thanks for the contribution, this looks solid. Merging now.