Tool system: Semantic tool retrieval (lazy tools with embeddings)

[Delqhi]

Problem

--lazy-tools reduces the manifest by simple keyword matching. The model still receives a flat list. SOTA systems use semantic tool retrieval: embed the task + tool descriptions, return only top-K relevant tools.

What to do

1. internal/toolretrieval/ package:
   • Embed tool descriptions (reuse internal/memory/embed.go or local embedding)
   • Index all tools at startup
   • Retrieve(task string, k int) []ToolSpec returns top-K relevant tools
2. sin-code chat --lazy-tools uses semantic retrieval instead of keyword filter
3. sin-code tool-search "auth module" — debug command
4. Cache embeddings in ~/.config/sin-code/tool-embeddings.bin
5. Include tool examples in embedding for better matching

Acceptance criteria

• Semantic tool retrieval works
• Top-K relevant tools returned
• Embedding cache persistent
• Debug command
• Better relevance than keyword matching