Replace library metaphors with verification language

[Aprasaks]

Context

Dechive is now centered on verification rather than a library metaphor. The UI still had visible language around shelves, librarians, and library framing.

Scope

• Replace library/librarian/shelf copy with verification, records, and guidance language.
• Update Deep Dive guide card and chat guide copy.
• Rename the Deep Dive fallback image reference to verification-oriented naming.
• Add an AGENTS note to keep future copy aligned with verification language.

Validation

• npm run build