fix(start-work): restore atlas-first slash discovery

Static slash-command discovery runs before agent registration, so /start-work regressed to Sisyphus even though config-time wiring still needed Atlas-aware fallback. Split builtin command resolution so discovery stays Atlas-first while command config remains availability-aware.

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

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

src/hooks/auto-slash-command/executor.test.ts

@@ -192,4 +192,24 @@ describe("auto-slash command executor plugin dispatch", () => {
     expect(result.replacementText).not.toContain("$ARGUMENTS")
     expect(result.replacementText).not.toContain("${user_message}")
   })
+
+  it("renders Atlas as the builtin start-work agent during slash-command execution", async () => {
+    // given
+
+    // when
+    const result = await executeSlashCommand(
+      {
+        command: "start-work",
+        args: "",
+        raw: "/start-work",
+      },
+      {
+        skills: [],
+      },
+    )
+
+    // then
+    expect(result.success).toBe(true)
+    expect(result.replacementText).toContain("**Agent**: atlas")
+  })
 })