Fix: Use user's selected model for tool selection + Add OpenRouter su…

[CG-8663]

…pport

Fixes #30: Tool selection now uses the user's selected model instead of hardcoded OpenAI gpt-5-mini.

Bug Fix:

• Remove hardcoded SMALL_MODEL constant from tool-executor.ts
• Pass model through ToolExecutorOptions interface
• Use this.model in selectTools() instead of hardcoded value

New Feature - OpenRouter Support:

• Add openrouter: prefix handler in llm.ts using OpenAI SDK
• Add OpenRouter provider to ModelSelector with popular models
• Add OPENROUTER_API_KEY to env.ts provider config

Additional:

• Fix incomplete MODEL_TO_PROVIDER_MAP in config.ts migration

Authored by CG-8663 utilising Claude Opus 4.5 and BMAD spec engineering