feat: implement story_name and subject

[lomefin]

Summary

• story_name — class-level setter/getter for a human-readable story description (avoids overriding Module#name)
• subject — takes a symbol and defines an instance method subject delegating to that attribute, so steps can refer to the main actor generically
• Updated README to use story_name instead of name
• Added 3 specs covering both methods

Test plan

• NamedStory.story_name returns the set string
• story_name returns nil when not set
• story.subject delegates to the named method