Rename project from Nexa to Osias

Renames the npm package (nexa-extension -> osias-extension), the
extension manifest name and Firefox gecko id
([email protected]), the internal runtime message-type
namespace and storage keys ('nexa:...' -> 'osias:...'), the
NexaMessage type, and all remaining Nexa branding in source comments,
UI copy, and CLAUDE.md to match the new osias.trade domain and the
osias-trade GitHub org.

No wire-protocol changes — purely internal naming plus user-facing
strings.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Tn7uDYjTuZEbLPwpEiSCUw

src/content-scripts/interventions/full-block.ts

@@ -9,7 +9,7 @@ export const fullBlock: Intervention = {
   id: 'full-block',
 
   apply(_container, _reason, _onLockedInteraction) {
-    console.warn('[nexa] full-block intervention is a v1 stub and is not yet implemented.');
+    console.warn('[osias] full-block intervention is a v1 stub and is not yet implemented.');
   },
 
   remove(_container) {},