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

README.md

@@ -2,7 +2,8 @@
 
 Flake-based NixOS config. Hosts: `jupiter` (ZimaBlade, NAS + services),
 `neptun` (netcup VPS: public reverse proxy, Authentik, headscale),
-`mercury` (Raspberry Pi 3B+, DNS/DHCP), `terra` (desktop).
+`mercury` (Raspberry Pi 3B+, DNS/DHCP), `terra` (desktop), `mars` (on-site,
+single-purpose: Hermes Agent only).
 
 ## Structure
 
@@ -26,6 +27,9 @@ hosts/
     vm.nix                 #   VirtualBox test image (jupiter-vbox)
   neptun/                     # netcup public reverse proxy + tailnet node
     configuration.nix  disk-config.nix  hardware-configuration.nix  secrets.nix
+  mars/                     # on-site, single-purpose: Hermes Agent only
+    configuration.nix  disk-config.nix  hardware-configuration.nix  secrets.nix
+    hermes-agent.nix          #   Hermes Agent (moved here from jupiter)
 secrets/                   # age-encrypted sops files, one per host
 scripts/                   # deploy, edit_secrets
 ```
@@ -84,6 +88,42 @@ an installer, partitions via disko, installs.
 Manual alternative (USB ISO): boot installer, `disko` the disk, then
 `nixos-install --flake .#jupiter`.
 
+## First install on mars
+
+mars is an older x86_64 box (unknown provenance, "got from work"), on-site,
+running Hermes Agent only (see `hosts/mars/hermes-agent.nix` — moved there
+from jupiter). Its age recipient, host key
+(`~/.config/homelab/mars/ssh_host_ed25519_key`), and `secrets/mars.yaml` are
+already set up, with `darman_password`/`samba_password`/`opencode_go_api_key`/
+`telegram_bot_token`/`hermes_dashboard_oidc_client_secret` carried over from
+jupiter's old instance. Two things are still placeholders and MUST be filled
+in before installing:
+
+1. **OS disk id** in `hosts/mars/disk-config.nix` (`ls -l /dev/disk/by-id`
+   once you have console/installer access on the box) — same `REPLACE-ME` in
+   `hosts/mars/configuration.nix`'s comment refers to the same disk, but only
+   `disk-config.nix`'s `device` actually needs editing (grub's own device list
+   comes from disko, see that file's comment).
+2. **`tailscale_authkey`** in `secrets/mars.yaml` — generate a fresh one
+   (see "Bootstrap the tailnet" under neptun below) rather than reusing an
+   old key; reusable pre-auth keys still expire.
+
+Boot mode is assumed **legacy BIOS** (grub, not systemd-boot) — unconfirmed;
+check `[ -d /sys/firmware/efi ]` once you're at the machine and see
+`hosts/mars/disk-config.nix`'s header comment if it turns out to be UEFI.
+
+Otherwise the flow is identical to the ZimaBlade steps above:
+```
+nix run github:nix-community/nixos-anywhere -- \
+  --flake .#mars \
+  --extra-files /tmp/extra \
+  --generate-hardware-config nixos-generate-config ./hosts/mars/hardware-configuration.nix \
+  --target-host root@<mars-ip>
+```
+(stage the host key into `/tmp/extra/etc/ssh/` first, same as step 4 there).
+Manual alternative (USB ISO): boot installer, `disko` the disk, then
+`nixos-install --flake .#mars`.
+
 ## First install on terra — no-USB findiso reinstall (replacing CachyOS)
 
 terra is a Ryzen 9 5900X / Radeon RX 6800 XT desktop, currently running
@@ -294,6 +334,20 @@ another way in.
   sudo tailscale logout && sudo systemctl restart tailscaled-autoconnect
   ```
 
+### mars
+
+- **Confirm the OIDC redirect still resolves.** hermes-agent.nix reuses
+  jupiter's old Authentik application (slug `hermes`, redirect
+  `https://hermes.mgaction.town/auth/callback`) unchanged — nothing to
+  reconfigure in Authentik, just verify `neptun`'s `hermes.mgaction.town`
+  vhost (now pointed at `mars.orbit.sol:9119`) actually reaches the
+  dashboard once mars is up and joined the tailnet.
+- **Carrying forward old chat history/memories:** mars starts with a fresh
+  Hermes state dir (`/var/lib/hermes/.hermes`). jupiter's old instance data
+  is backed up at `/mnt/data/AppData/hermes.bak-2026-08-21` — rsync it over
+  (via the `/mnt/jupiter` samba mount) before the first switch if you want
+  it preserved instead of starting clean.
+
 ### mercury (Raspberry Pi 3B+)
 
 - `./deploy flash mercury /dev/sdX` writes the dedicated age key to the root