Artifacts gain a 'quickstart' action that replaces several separate setup calls

What

• A new action: "quickstart" for the Artifacts tool looks up published Artifact types, lists the account's design systems, and, for slides or other design-related artifacts, fetches and attaches the default design system's README.md into the conversation, with permission-rule checks and size limits.
• It replaces the previous multi-step flow of listing types, listing design systems, reading the default design system's README, and, for a plain page, separately loading the artifact-design skill, collapsing them into one call with an intent parameter. It also adds guidance that steers Claude-Docs-type requests toward a first-party connector instead of an Artifact.
• If quickstart has already run once in a conversation, later calls are told to skip re-listing design systems.
• A new status case renders a human-readable summary of what the quickstart flow did: which artifact type matched, whether type listing was skipped by a permission rule, whether document-destination or page-design guidance was given, and how many design systems were listed or whether their guide was read.
• Trying to pass a design_systems field to a plain artifact-type create is now rejected, with an explanation that it belongs to the quickstart action instead.
• The whole feature is gated behind three flags being on together (an internal type-catalog flag, a type-create flag, and CLAUDE_CODE_ARTIFACT_QUICKSTART / the tengu_cobalt_plinth_woad gate), all off by default.

Why This cuts down the number of round-trips Claude needs to set up a new artifact of a known type, like slides, since it no longer has to separately list types, list design systems, and read the README before it can start.