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/tools/slashcommand/command-discovery.test.ts

@@ -255,4 +255,15 @@ Use nested command.
     expect(nestedCommand?.content).toContain("Use nested command.")
     expect(nestedCommand?.scope).toBe("opencode-project")
   })
+
+  it("keeps builtin start-work routed to Atlas during static discovery", () => {
+    // given
+
+    // when
+    const commands = discoverCommandsSync(projectDir)
+    const startWorkCommand = commands.find((command) => command.name === "start-work")
+
+    // then
+    expect(startWorkCommand?.metadata.agent).toBe("atlas")
+  })
 })