Command renamed to names-generator

• Install the command as names-generator, matching the distribution name, so uvx names-generator works
• Name the program after the canonical command in its usage text
• Refer to names-generator throughout the documentation

The generate_name command is still installed and behaves identically, so existing
scripts and shell aliases continue to work. Note that only the hyphenated spelling
resolves as a command; uvx names_generator will not find it.