Refresh StarryKit Plugin user experience

[Mine77]

What changed

• replace the developer-oriented README with bilingual user-facing product guides
• add StarryKit brand assets and real product demo media
• adapt the canonical Authoring Skill from the main-agent design direction to the public MCP tools
• replace the legacy hosts compatibility layer with bilingual manual setup docs
• simplify contract validation into focused plugin bundle integrity tests

Why

The repository should be the installable, user-facing home of StarryKit Plugin rather than an exported engineering snapshot.

Validation

• npm test (5/5 passing)
• git diff --check