docs: condense comments across the repo

Comments had drifted into multi-paragraph narrative (git commit
lineage, debugging stories, restated code) in several hot spots
(scripts/deploy, hermes-agent.nix, flake.nix, gitea.nix, headscale.nix).
Trim every comment to its load-bearing "why" — gotchas, safety
warnings, and non-obvious rationale survive verbatim in substance,
just tightened to 1-2 sentences; historical narrative and anything
already covered in CLAUDE.md is cut. No code/logic changed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UJqEmY1y3AYX3JoX4Y6b21

hosts/mars/luna-sites-test.nix

@@ -29,9 +29,9 @@ let
     cmd = [ "/bin/sleep" "infinity" ];
   };
 
-  # The "app" luna builds on top of. No network in the VM, so it is loaded
-  # from the store instead of pulled. Runs under luna-apps, which has no
-  # /nix/store mount — hence the closure inside the image.
+  # The "app" luna builds on top of, loaded from the store since the VM has
+  # no network. Runs under luna-apps, which has no /nix/store mount — hence
+  # the closure baked into the image.
   app = busyboxImage {
     name = "testapp";
     extraCommands = "mkdir -p www && echo hello > www/index.html";