Let users rename a skill in the skills store

Problem Once a skill exists in the skills store, its name is fixed. The skill form disables the name field with "Skill name cannot be changed after creation", and the API rejects a changed name on publish with the same message (LLMSkillSerializer.validate). The only way to fix a typo, or to rename a skill after its purpose drifted, is Duplicate under a new name and then Archive the original. That loses the version history, resets the owners to whoever duplicated it, and leaves a second entry in the store until someone archives the old one. The name matters more than a label would: it is the slug agents call (skill-get, /posthog-skill-store:<name>), the URL (/skills/<name>), and the directory in the zip export and the Claude Code plugin marketplace. So people want to get it right, and today a wrong name is permanent. #91634 covers picking a name when installing from the community store and explicitly leaves post-creation rename out of scope. This issue is that …