feat(work): headless Secret Service for gcx keychain tokens

gcx stores its OAuth access and refresh tokens in the system keychain
unconditionally -- its config file keeps only opaque `keychain:gcx:v2:...`
handles -- and exposes no plaintext fallback. With nothing owning
org.freedesktop.secrets on this headless WSL box, `gcx login` authenticates
against Grafana and then dies writing its config: "The name is not activatable".

Add services.headlessSecretService: gnome-keyring as a systemd --user service,
unlocking the login keyring at start. home-manager's own services.gnome-keyring
does not fit here on two counts -- it is WantedBy graphical-session-pre.target,
which never activates without a desktop session, and it passes no --unlock, so
writes would block on a GUI prompter that does not exist.

Only the secrets component is started. The ssh component is deliberately off: it
would claim SSH_AUTH_SOCK and displace services.ssh-agent, breaking SSH auth and
signed commits.

The unlock password defaults to a random one generated on first activation under
$XDG_DATA_HOME. The passwordFile option is the seam for supplying it from an
agenix secret instead, once that lands.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

users/emmathorpe/work.nix

@@ -49,6 +49,13 @@
     pkgs.gcx # Grafana Cloud CLI (dashboards, SLOs, synthetics, alerts)
   ];
   services.ssh-agent.enable = true;
+
+  # gcx (above) keeps its OAuth tokens in the system keychain and has no
+  # plaintext fallback, so this WSL box needs something owning
+  # org.freedesktop.secrets. See home/secret-service.nix for why
+  # home-manager's services.gnome-keyring cannot be used on a headless host,
+  # and for the security trade-off of an auto-unlocked keyring.
+  services.headlessSecretService.enable = true;
   home.shellAliases = {
     docker = "/run/current-system/sw/bin/docker";
   };