docs(plugin): spell out what blurBookingCodes does, and preview it

The context table listed the new field without saying what a plugin is meant to
do with it. It is a display hint — booking codes still arrive in full over
trek:invoke — and reading it as redaction is the one mistake worth preventing.

The dev preview hardcoded it to false, so the blurred path was the only host flag
an author could not try out; it gets a toggle like reduce motion and the rest.

plugin-sdk/src/cli/dev.ts

@@ -634,6 +634,7 @@ iframe{width:100%;border:0;background:transparent;min-height:120px;display:block
   <label>Accent <select id="accent"><option value="default">default</option><option value="indigo">indigo</option><option value="teal">teal</option><option value="rose">rose</option></select></label>
   <label><input type="checkbox" id="rm"> reduce motion</label>
   <label><input type="checkbox" id="nt"> no transparency</label>
+  <label><input type="checkbox" id="blur"> blur booking codes</label>
   <label><input type="checkbox" id="trip" checked> trip context</label>
 </header>
 <div class="stage"><div class="wrap"><iframe id="f" src="/ui/index.html" sandbox="allow-scripts allow-forms" referrerpolicy="no-referrer" title="${id}"></iframe></div></div>
@@ -658,7 +659,7 @@ function ctx(){
     tripId: val("trip").checked?${tripId}:null, userId:"1",
     user:{name:"Dev User",avatar:null,isAdmin:true},
     appearance:{scheme:accent,density:"comfortable",reducedMotion:val("rm").checked,noTransparency:val("nt").checked},
-    formats:{locale:"en",currency:"EUR",timeFormat:"24h",distanceUnit:"metric",temperatureUnit:"celsius",timezone:Intl.DateTimeFormat().resolvedOptions().timeZone,blurBookingCodes:false},
+    formats:{locale:"en",currency:"EUR",timeFormat:"24h",distanceUnit:"metric",temperatureUnit:"celsius",timezone:Intl.DateTimeFormat().resolvedOptions().timeZone,blurBookingCodes:val("blur").checked},
     tokens:tokens};
 }
 function postCtx(){ if(f.contentWindow) f.contentWindow.postMessage(ctx(),"*"); }
@@ -698,7 +699,7 @@ window.addEventListener("message", function(ev){
   }
 });
 var geoTick=null;
-["theme","accent","rm","nt","trip"].forEach(function(id){ val(id).addEventListener("change",postCtx); });
+["theme","accent","rm","nt","blur","trip"].forEach(function(id){ val(id).addEventListener("change",postCtx); });
 f.addEventListener("load", function(){ f.style.height="120px"; postCtx(); });
 var __v; setInterval(function(){ fetch("/__dev/version").then(function(r){return r.text();}).then(function(v){ if(__v&&v!==__v){ f.src=f.src; } __v=v; }).catch(function(){}); },1000);
 </script>