Feat v0.2.9 builtin extension retirement

Stop auto-seeding 6 chat-centric extensions on startup. Move them to
standard packages with js/ layout and "requires": ["chat"] metadata.
Delete SeedBuiltinPackages, seed tests, Dockerfile COPY, and
extensions/builtin/ directory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/main.go

@@ -116,8 +116,6 @@ func main() {
 		// Seed additional users from env (dev/test only, skipped in production)
 		handlers.SeedUsers(cfg, stores, uekCache)
 
-		// Seed builtin extensions from disk (idempotent, version-aware)
-		handlers.SeedBuiltinPackages(stores, "extensions/builtin")
 
 	}
 	defer database.Close()