terra: kexec-local hangs hard on real hardware, switch docs to USB installer

Confirmed on real hardware: kexec's device_shutdown() pass runs (SCSI disks
sync fine in the log) then the machine goes dark for good — journalctl
--list-boots showed a ~15min gap before the next boot, a genuine hang needing
a manual power cycle, not a slow jump. Near-certainly amdgpu (RX 6800 XT):
discrete AMD GPUs are known to hang during kexec's device-shutdown pass with
no clean handoff before the jump, same class of issue as jupiter's
reboot=pci warm-reboot workaround, just fatal here instead of slow.

README's terra install section now leads with the USB installer path instead
(build ISO, dd to USB, rsync the repo over, disko + nixos-install locally).
CLAUDE.md's gotchas list gets the same warning. installer-iso is renamed from
jupiter-installer to homelab-installer since it's genuinely host-agnostic,
and now ships git.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

flake.nix

@@ -163,20 +163,22 @@
         };
 
         # Bootable USB recovery installer with our SSH key + sshd + DHCP.
+        # Reusable for any host's manual-USB install path (jupiter, terra, ...).
         # Build the ISO:
         #   nix build .#nixosConfigurations.installer-iso.config.system.build.isoImage
-        # dd it to a USB stick, boot the ZimaBlade from it, SSH in, ./deploy install.
+        # dd it to a USB stick, boot the target from it, SSH in, ./deploy install.
         installer-iso = nixpkgs.lib.nixosSystem {
           inherit system;
           modules = [
             (nixpkgs + "/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix")
-            ({ ... }: {
+            ({ pkgs, ... }: {
               services.openssh.enable = true;
               services.openssh.settings.PermitRootLogin = "prohibit-password";
               users.users.root.openssh.authorizedKeys.keys = [
                 "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGZpkPVhzi1zG5JI9hWyUgdyvNIQbp4ts4jw3idpMhhN erik@laptop"
               ];
-              networking.hostName = "jupiter-installer";
+              networking.hostName = "homelab-installer";
+              environment.systemPackages = [ pkgs.git ];
             })
           ];
         };