feat(secrets): scaffold sops-nix for declarative secrets

Wires sops-nix as a flake input and bakes the NixOS module into
configuration.nix via modules/secrets.nix. Per-host defaults live in
modules/secrets.nix:

- defaultSopsFile = ../secrets/${settings.hostName}.yaml
- defaultSopsFormat = yaml
- age.keyFile = /home/${settings.user}/.config/sops/age/keys.txt

The whole sops block is gated on `builtins.pathExists` so flake eval
succeeds before the encrypted file is created — important during the
scaffold-bootstrap phase where the consumer hasn't yet generated an
age key.

Adds .sops.yaml with a placeholder admin recipient (overwrite with
your real age public key before encrypting anything) and a
creation_rules block matching `secrets/*.yaml`.

.gitignore loosened so `secrets/*.yaml` and `secrets/README.md` can
be checked in while plaintext key material (`*.key`, `*.pem`) and
anything else under `secrets/` stays ignored. The pre-commit secret
scanner most consumers use is the second line of defense.

secrets/README.md documents the workflow at the directory level.
The substantive beginner walkthrough lands in a follow-up commit at
docs/secrets-management.md.

`nix flake check --no-build` stays green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

modules/secrets.nix

@@ -0,0 +1,42 @@
+# sops-nix per-host wiring.
+# pragma: allowlist secret start
+#
+# Imports the sops-nix NixOS module and points it at this host's
+# encrypted file + the consumer's age private key.
+#
+# - Recipients (which age public keys can decrypt) are declared in
+#   `.sops.yaml` at the repo root.
+# - The encrypted file for this host lives at
+#   `secrets/${settings.hostName}.yaml`. Create it with:
+#       sops secrets/${settings.hostName}.yaml
+#   sops auto-encrypts on save using the recipients from .sops.yaml.
+# - The matching private key lives at
+#   `/home/${settings.user}/.config/sops/age/keys.txt`. Generate it
+#   one-time with `age-keygen -o ~/.config/sops/age/keys.txt`.
+#
+# The whole sops block is gated on `builtins.pathExists` so flake
+# eval succeeds before the encrypted file exists — useful for the
+# scaffold-bootstrap phase. See `docs/secrets-management.md` for a
+# walkthrough.
+# pragma: allowlist secret end
+{
+  config,
+  lib,
+  pkgs,
+  inputs,
+  settings,
+  ...
+}:
+
+let
+  sopsFile = ../secrets/${settings.hostName}.yaml;
+in
+{
+  imports = [ inputs.sops-nix.nixosModules.sops ];
+
+  sops = lib.mkIf (builtins.pathExists sopsFile) {
+    defaultSopsFile = sopsFile;
+    defaultSopsFormat = "yaml";
+    age.keyFile = "/home/${settings.user}/.config/sops/age/keys.txt";
+  };
+}