Summary

• When sandbox exec detects a missing .git directory during sync, the sandbox is now stopped and the session removed from storage
• Previously the sandbox was left running, requiring a manual rwx sandbox reset before retrying
• Uses a sentinel error (ErrSandboxNoGitDir) so ExecSandbox can identify the specific failure and clean up via the already-open SSH connection
• We can use this for other cases as we discover them where an error should kill a sandbox (i.e. possibly the sandbox run definition file becoming dirty)

Test plan

• Existing test updated to verify __rwx_sandbox_end__ is sent and session is removed from storage
• All other sandbox tests continue to pass
• Manual: configure a sandbox without preserve-git-dir: true, run rwx sandbox exec, confirm it errors and the sandbox is stopped (next exec creates a fresh one)

╰─⠠⠵ ~/code/cli/rwx sandbox exec -- pwd
Started sandbox: fb6726b9d3d549d68322e3f9b7579363
https://cloud.rwx.com/mint/rwx/runs/fb6726b9d3d549d68322e3f9b7579363
Error: failed to sync changes to sandbox: no .git directory found in sandbox. Set 'preserve-git-dir: true' on your git/clone task

# Fix it, try again, note the new sandbox URL below


╰─⠠⠵ ~/code/cli/rwx sandbox exec -- pwd
Started sandbox: 50e3de01ad524efcaf1300b33c11030c
https://cloud.rwx.com/mint/rwx/runs/50e3de01ad524efcaf1300b33c11030c
/var/mint-workspace
No changes to pull from sandbox.