Fix frozen command_id default causing concurrent execution stdout corruption

[dines-rl]

Summary

• execute and execute_and_await_completion (both sync and async) used command_id: str = str(uuid7()) as a default parameter — Python evaluates this once at class definition time, so every call without an explicit command_id reused the same frozen UUID
• When multiple concurrent calls hit the same devbox with the same command_id, the rage agent creates two executions writing to the same output file (/runloop/commands/{uuid}.stdout), producing garbled/interleaved stdout
• Fix: change the default to None and generate a fresh uuid7() per call in the method body

Root cause chain

1. SDK: frozen default command_id → all concurrent calls share one UUID
2. Rage agent: no duplicate command_id guard — HashMap::insert silently overwrites the prior entry
3. Rage agent: File::create on the same path truncates the existing stdout file while the first process is still writing → interleaved bytes

Test plan

• SDK unit tests pass (they always pass explicit command_id=)
• Smoketest CI run with cooperative async concurrency (the scenario that triggered this)