fix(agent-loader): convert model object to string for opencode compatibility

mapClaudeModelToOpenCode() returns {providerID, modelID} but opencode
expects model as a string. Both agent loaders now convert to
'providerID/modelID' string format before assigning to config.

src/features/claude-code-agent-loader/loader.ts

@@ -43,12 +43,15 @@ function loadAgentsFromDir(agentsDir: string, scope: AgentScope): LoadedAgent[]
        const formattedDescription = `(${scope}) ${originalDescription}`
 
        const mappedModelOverride = mapClaudeModelToOpenCode(data.model)
+       const modelString = mappedModelOverride
+         ? `${mappedModelOverride.providerID}/${mappedModelOverride.modelID}`
+         : undefined
 
        const config: ClaudeCodeAgentConfig = {
          description: formattedDescription,
          mode: data.mode || "subagent",
          prompt: body.trim(),
-         ...(mappedModelOverride ? { model: mappedModelOverride } : {}),
+         ...(modelString ? { model: modelString } : {}),
        }
 
        const toolsConfig = parseToolsConfig(data.tools)

src/features/claude-code-plugin-loader/agent-loader.ts

@@ -47,12 +47,15 @@ export function loadPluginAgents(plugins: LoadedPlugin[]): Record<string, Claude
         const formattedDescription = `(plugin: ${plugin.name}) ${originalDescription}`
 
         const mappedModelOverride = mapClaudeModelToOpenCode(data.model)
+        const modelString = mappedModelOverride
+          ? `${mappedModelOverride.providerID}/${mappedModelOverride.modelID}`
+          : undefined
 
         const config: ClaudeCodeAgentConfig = {
           description: formattedDescription,
           mode: "subagent",
           prompt: body.trim(),
-          ...(mappedModelOverride ? { model: mappedModelOverride } : {}),
+          ...(modelString ? { model: modelString } : {}),
         }
 
         const toolsConfig = parseToolsConfig(data.tools)