Merge origin/main into claude/project-thread-g33c7k

The PR conflicted with main in three places, each a count this branch
had moved and main made count-free: the metadata registry type count in
concepts/metadata-lifecycle.mdx, the schema counts in two
quick-reference headings, and the closed/total pin in
kernel/metadata-type-schemas.test.ts (main computes the open set from
the registry instead). Main's side is taken in all three. The platform
skill sits one token over its ceiling after main's change, so its
approvals sentence loses two words.

Claude-Session: https://claude.ai/code/session_01GTntg542RUmmRRbMY6wMZD
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude