fix: register custom user agents in delegate-task resolver (#2689)

agent-config-handler.ts now registers custom agents from
~/.config/opencode/agents/ into the task subagent registry.

5005 tests pass, tsc clean.

Closes #2689

src/plugin-handlers/agent-config-handler.test.ts

@@ -293,6 +293,81 @@ describe("applyAgentConfig builtin override protection", () => {
     expect(createSisyphusJuniorAgentSpy).toHaveBeenCalledWith(undefined, "openai/gpt-5.4", false)
   })
 
+  test("defaults mode to subagent for configAgent entries missing mode", async () => {
+    // given
+    const config = createBaseConfig()
+    ;(config as Record<string, unknown>).agent = {
+      "custom-reviewer": {
+        name: "custom-reviewer",
+        prompt: "Review code for security issues",
+        description: "Custom code reviewer",
+      },
+    }
+
+    // when
+    const result = await applyAgentConfig({
+      config,
+      pluginConfig: createPluginConfig(),
+      ctx: { directory: "/tmp" },
+      pluginComponents: createPluginComponents(),
+    })
+
+    // then
+    const customAgent = result["custom-reviewer"] as Record<string, unknown>
+    expect(customAgent).toBeDefined()
+    expect(customAgent.mode).toBe("subagent")
+  })
+
+  test("preserves explicit mode on configAgent entries", async () => {
+    // given
+    const config = createBaseConfig()
+    ;(config as Record<string, unknown>).agent = {
+      "custom-primary": {
+        name: "custom-primary",
+        prompt: "Primary agent",
+        mode: "primary",
+      },
+    }
+
+    // when
+    const result = await applyAgentConfig({
+      config,
+      pluginConfig: createPluginConfig(),
+      ctx: { directory: "/tmp" },
+      pluginComponents: createPluginComponents(),
+    })
+
+    // then
+    const customAgent = result["custom-primary"] as Record<string, unknown>
+    expect(customAgent).toBeDefined()
+    expect(customAgent.mode).toBe("primary")
+  })
+
+  test("defaults mode to subagent for plugin agents missing mode", async () => {
+    // given
+    const pluginComponents = createPluginComponents()
+    pluginComponents.agents = {
+      "plugin-worker": {
+        name: "plugin-worker",
+        prompt: "Do work",
+        description: "Plugin worker agent",
+      } as Record<string, unknown>,
+    }
+
+    // when
+    const result = await applyAgentConfig({
+      config: createBaseConfig(),
+      pluginConfig: createPluginConfig(),
+      ctx: { directory: "/tmp" },
+      pluginComponents,
+    })
+
+    // then
+    const pluginAgent = result["plugin-worker"] as Record<string, unknown>
+    expect(pluginAgent).toBeDefined()
+    expect(pluginAgent.mode).toBe("subagent")
+  })
+
   test("includes project and global .agents skills in builtin agent awareness", async () => {
     // given
     const projectAgentsSkill = {