Ship a Logbuch skill and record token use

- docs/skill.md teaches a Claude what Logbuch is for, how an entry sounds
  and in which order the calls go, served at GET /api/v1/skill and as a
  download next to the spec and the guide
- a test keeps the skill from drifting: frontmatter, only endpoints that
  exist, house rules
- resolveClient stamps last_used_at, the access list said never used even
  after eleven entries

src/lib/admin-routes.ts

@@ -12,7 +12,7 @@ export const adminClientsPath = '/admin/clients' as Route
 export const adminPostTypesPath = '/admin/post-types' as Route
 export const adminNewPostTypePath = '/admin/post-types/new' as Route
 
-export function adminApiFilePath(file: 'openapi' | 'style-guide'): Route {
+export function adminApiFilePath(file: 'openapi' | 'style-guide' | 'skill'): Route {
 	return `/admin/api/files/${file}` as Route
 }