fix: remove committed binary and stop leaking API key in prompt

Remove cmd/supervisor/supervisor binary from git (was accidentally
committed) and add it to .gitignore. Move LITELLM_API_KEY from the
prompt string into the subprocess env, preventing it from appearing
in error log output when JSON parsing fails.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -4,6 +4,7 @@
 # Binaries
 bin/
 *.exe
+cmd/supervisor/supervisor
 
 # Go
 vendor/