Replace the YAML exercise catalogs with the motion-rig exercise library

The exercise picker now lists the bundled exercise library — the
motion-rig exercises exported from Exercise Library/ — instead of the
two *.exercises.yaml starter catalogs. ExerciseListLoader and the Yams
dependency are removed; ExerciseMotionLibrary gains exerciseNames, the
bundle enumeration the picker reads.

Workouts/ExerciseFigure/ExerciseMotion.swift

@@ -83,6 +83,19 @@ enum ExerciseMotionLibrary {
         let body: ExerciseBodyProfile
     }
 
+    /// Every exercise with a bundled motion script, sorted alphabetically. XcodeGen
+    /// flattens resource groups, so `<Exercise Name>.motion.json` files land in the
+    /// bundle root alongside `body.json` — enumerate all json and filter on the
+    /// compound suffix (which `body.json` doesn't match).
+    static let exerciseNames: [String] = {
+        let urls = Bundle.main.urls(forResourcesWithExtension: "json", subdirectory: nil) ?? []
+        return urls
+            .map(\.lastPathComponent)
+            .filter { $0.hasSuffix(".motion.json") }
+            .map { String($0.dropLast(".motion.json".count)) }
+            .sorted()
+    }()
+
     /// The motion script plus the neutral body profile for `exerciseName`, or `nil`
     /// when no bundled motion matches (most exercises have none — the caller keeps
     /// its space empty).