add mars host, move Hermes Agent there from jupiter

New on-site host mars runs Hermes Agent as its sole service: joins the
tailnet, mounts jupiter's samba share at /mnt/jupiter (doubling as
Hermes's shared dropbox), and hosts state locally under /var/lib/hermes.
Same Authentik OIDC app/Telegram bot as before, just relocated — neptun's
hermes.mgaction.town vhost now points at mars.orbit.sol instead of jupiter.

hosts/jupiter/hermes-agent.nix and its three sops secrets are removed;
jupiter's Caddy vhost for it is gone too. Also refreshes tailscale_authkey
across all hosts and fixes two stale "erik@laptop" keys in flake.nix's
kexec/installer-iso images (leftover from a previous laptop, already
swapped out of common.nix back in 2fd5752) to darman@terra.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FHr5ug9pu8q4XPrRkFnzJ

hosts/neptun/configuration.nix

@@ -110,6 +110,13 @@
     reverse_proxy http://jupiter.orbit.sol:2283
   '';
 
+  # ---- Hermes dashboard ----
+  # Authentik-gated (hosts/mars/hermes-agent.nix has the OIDC config and the
+  # "create the Authentik app" instructions — moved here from jupiter).
+  services.caddy.virtualHosts."hermes.mgaction.town".extraConfig = ''
+    reverse_proxy http://mars.orbit.sol:9119
+  '';
+
   # ---- Gitea WebUI ----
   # Gitea's web UI and HTTPS clones (services/dev/gitea.nix, HTTP_PORT 3000).
   # Its SSH side is the separate :2222 forward further down.