fix(agents): use null-prototype accumulator to prevent __proto__ pollution

Addresses cubic-dev-ai P1 review: Object.hasOwn() alone doesn't prevent
a crafted '__proto__' agent name from mutating the result object's
prototype chain. Using Object.create(null) eliminates inherited properties
entirely, making both the hasOwn checks and property assignments safe.

src/features/claude-code-agent-loader/opencode-config-agents-reader.ts

@@ -79,7 +79,7 @@ function convertInlineAgent(agentData: unknown): ClaudeCodeAgentConfig | null {
 }
 
 export function readOpencodeConfigAgents(directory: string): Record<string, ClaudeCodeAgentConfig> {
-  const result: Record<string, ClaudeCodeAgentConfig> = {}
+  const result: Record<string, ClaudeCodeAgentConfig> = Object.create(null)
 
   for (const configPath of getConfigPaths(directory)) {
     try {