Fully implement subcommand-specific contexts in CommandManager

[coderabbitai]

Description

Currently, CommandManager collects contexts from all subcommands and applies them to the parent command:

// TODO: Implement subcommand-specific contexts
InteractionContextType[] contexts = getSubcommandContexts(entry.getValue());
SlashCommandData d = Commands.slash(entry.getKey(), entry.getKey()).setContexts(contexts);

This works as a basic implementation, but individual subcommands may need their own specific context settings.

Proposed Solution

Implement a more granular approach that allows specifying different contexts for individual subcommands rather than just aggregating them at the parent command level.

References

• Discussed in PR: Move to using JSON config for guilds. #41 (comment)