Reworked guidance for design/slides Artifact storage instructions

What

The logic that generates instructions for design, slides, and design-system Artifacts about where their content actually lives was reworked.

• A new lookup table (keyed by design, slides, design-system) and helper functions generate instructions explaining whether an Artifact's content lives in its own published files or in its database-style store, based on an index marker file (canvas.json, deck.json, or design-system.json).
• A migration note clarifies that a design-system's content can move from the database store to files the first time (or any time) it's opened, and never moves back.
• The write-guidance text shown when reading an Artifact that declares write_db was rewritten: it now factors in file-list instructions and produces different text depending on whether the type is file-based, plus trailing instructions about published files.

Why

This should make Claude's guidance about where to write changes to a design/slides Artifact more accurate, since the storage model (files vs. database) can differ by type and can change over an Artifact's lifetime.