Bug Description:
The codesearch tool in opencode calls get_code_context_exa on the Exa MCP server, but this tool does not exist.

Steps to Reproduce:

1. Set OPENCODE_ENABLE_EXA=1 and EXA_API_KEY=
2. Try to use the codesearch tool

Actual Behavior:
MCP error -32602: Tool get_code_context_exa not found

Investigation:
Querying tools/list on Exa MCP shows only 2 available tools:

• web_search_exa (works fine)
• web_fetch_exa

get_code_context_exa referenced in packages/opencode/src/tool/codesearch.ts:44 does not exist.