Make explore agent to be opencode/grok-code

Grok Code is SuperFast and free at the moment.
For the purpose of exploring existing codebase,
grok code's intelligence is just enough for that. Plus Superfast.

src/agents/explore.ts

@@ -4,7 +4,7 @@ export const exploreAgent: AgentConfig = {
   description:
     'Fast agent specialized for exploring codebases. Use this when you need to quickly find files by patterns (eg. "src/components/**/*.tsx"), search code for keywords (eg. "API endpoints"), or answer questions about the codebase (eg. "how do API endpoints work?"). When calling this agent, specify the desired thoroughness level: "quick" for basic searches, "medium" for moderate exploration, or "very thorough" for comprehensive analysis across multiple locations and naming conventions.',
   mode: "subagent",
-  model: "anthropic/claude-haiku-4-5",
+  model: "opencode/grok-code",
   temperature: 0.1,
   tools: { write: false, edit: false },
   prompt: `You are a file search specialist. You excel at thoroughly navigating and exploring codebases.