fix(start-work): restore atlas native command routing

Route the builtin /start-work command to Atlas when Atlas is available so OpenCode resolves the native command agent correctly before plugin hooks run.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/builtin-commands/commands.test.ts

@@ -1,8 +1,17 @@
-import { describe, test, expect } from "bun:test"
+import { afterEach, beforeEach, describe, test, expect } from "bun:test"
 import { loadBuiltinCommands } from "./commands"
 import { HANDOFF_TEMPLATE } from "./templates/handoff"
 import { REMOVE_AI_SLOPS_TEMPLATE } from "./templates/remove-ai-slops"
 import type { BuiltinCommandName } from "./types"
+import { _resetForTesting, registerAgentName } from "../claude-code-session-state"
+
+beforeEach(() => {
+  _resetForTesting()
+})
+
+afterEach(() => {
+  _resetForTesting()
+})
 
 describe("loadBuiltinCommands", () => {
   test("should include handoff command in loaded commands", () => {
@@ -69,6 +78,17 @@ describe("loadBuiltinCommands", () => {
     //#then
     expect(commands["start-work"].agent).toBe("sisyphus")
   })
+
+  test("should preassign Atlas as the native agent for start-work when Atlas is registered", () => {
+    //#given
+    registerAgentName("atlas")
+
+    //#when
+    const commands = loadBuiltinCommands()
+
+    //#then
+    expect(commands["start-work"].agent).toBe("atlas")
+  })
 })
 
 describe("loadBuiltinCommands — remove-ai-slops", () => {