docs: update Loki queries from host to hostname label

Update all LogQL examples, agent instructions, and scripts to use
the hostname label instead of host, matching the Prometheus label
naming convention. Also update pipe-to-loki and bootstrap scripts
to push hostname instead of host.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hosts/template2/bootstrap.nix

@@ -28,7 +28,7 @@ let
             streams: [{
               stream: {
                 job: "bootstrap",
-                host: $host,
+                hostname: $host,
                 stage: $stage,
                 branch: $branch
               },