feat: add okdev exec script mode

[acmore]

Summary

• add local-file okdev exec --script <file> support for fanout and detach flows
• preserve exact argv for detached exec instead of flattening commands back into one shell string
• document the new exec behavior and add coverage for script upload, shebang handling, and detached argv preservation

Verification

• go test ./internal/cli
• .venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt