fix(start-work): restore atlas handoff

Keep native /start-work resolvable on Sisyphus, but switch the work session back to Atlas when Atlas is registered. Stamp the outgoing agent with Atlas's actual list-display key so config→start-work execution resolves correctly and still falls back to Sisyphus when Atlas is unavailable.

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

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

src/features/builtin-commands/commands.ts

@@ -58,7 +58,7 @@ ${REFACTOR_TEMPLATE}
   },
   "start-work": {
     description: "(builtin) Start Sisyphus work session from Prometheus plan",
-    agent: "atlas",
+    agent: "sisyphus",
     template: `<command-instruction>
 ${START_WORK_TEMPLATE}
 </command-instruction>