Resuming an in-process teammate now validates its recorded name more strictly

When Claude Code resumes an in-process teammate (a subagent running in the same process), it now checks the teammate's recorded name more strictly: the name must be a non-empty string matching a specific pattern and must not look like an agent ID. Previously this check only screened out reserved names. Names that fail now produce a clearer error explaining the name is reserved, looks like an agent ID, or isn't a valid name.