Document & enforce valid characters for all configuration

It would be exceptionally useful to document the valid characters for all configurations of SAP (e.g., long & short flag names, long & short option names, command names, etc.), and enforce rejection of all invalid characters in one central location.

This will:

- prevent different parts of SAP from expecting different sets of valid characters for the same values
- simplify shell completion script generation
- prevent bugs

No files or tests are named. Start by locating the configuration paths for long and short flag names, long and short option names, and command names, then identify where their validation can be centralized. Done means the valid characters are documented for each configuration and invalid characters are consistently rejected.

Written by the indexing model from the issue text.