Sync Shadcn

Handle $ARGUMENTS.

Goal: compare the tracked upstream shadcn docs baseline with the current ../shadcn/apps/v4 target, inventory every added/modified/deleted upstream change, map each change to Plate's docs app, classify the merge decision, write a reviewable plan under docs/sync/shadcn, directly merge any qualifying tiny overlap fixes, then stop for user review of the remaining slices. A later explicit user acceptance starts implementation mode for a named plan and slice.

If $ARGUMENTS starts with a command name, dispatch to that command before the default planning/implementation flow. If $ARGUMENTS names a feature, product surface, or slice, run a scoped planning lane for that scope. The default full-range planning lane applies only when no command or scope is mentioned.