[FEATURE]:add Ninelayer as a web search provider #33209

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Describe the enhancement you want to request

Summary

Add Ninelayer (https://ninelayer.in) as a third local web search provider alongside Exa and Parallel.

Motivation

Ninelayer is an agent-native search engine optimized for AI consumption — structured results,
low token overhead (~1,500 tokens per query), and ~97% first-shot resolution. It exposes a
stateless HTTP endpoint following the same MCP tools/call contract that Exa and Parallel use,
making it a natural fit for the existing websearch provider pattern.

What this adds

• OPENCODE_ENABLE_NINELAYER=true flag to enable Ninelayer as the search provider
• NINELAYER_API_KEY env var for authentication
• OPENCODE_WEBSEARCH_PROVIDER=ninelayer for explicit override
• Maps the existing type parameter (auto/fast/deep) to Ninelayer's retrieval_mode (balance/focused/explore)

What it does NOT change

• Exa and Parallel are unaffected
• No changes to the permission system, agent config, or tool registry
• Follows the exact same pattern as the existing providers

Testing

Verified end-to-end with the stateless endpoint returning results in ~1.3s round-trip.