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

@@ -69,12 +69,22 @@ describe("loadBuiltinCommands", () => {
     expect(commands.handoff.description).toContain("context summary")
   })
 
-  test("should preassign Sisyphus as the native agent for start-work", () => {
+  test("should default start-work to Atlas for static slash-command discovery", () => {
     //#given - no disabled commands
 
     //#when
     const commands = loadBuiltinCommands()
 
+    //#then
+    expect(commands["start-work"].agent).toBe("atlas")
+  })
+
+  test("should preassign Sisyphus as the native agent for start-work when command config checks registered agents", () => {
+    //#given - no atlas registration
+
+    //#when
+    const commands = loadBuiltinCommands(undefined, { useRegisteredAgents: true })
+
     //#then
     expect(commands["start-work"].agent).toBe("sisyphus")
   })
@@ -84,7 +94,7 @@ describe("loadBuiltinCommands", () => {
     registerAgentName("atlas")
 
     //#when
-    const commands = loadBuiltinCommands()
+    const commands = loadBuiltinCommands(undefined, { useRegisteredAgents: true })
 
     //#then
     expect(commands["start-work"].agent).toBe("atlas")