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>

.sops.yaml

@@ -0,0 +1,21 @@
+# sops recipient declarations.
+#
+# Replace the placeholder below with YOUR age public key before
+# encrypting any files. One-time setup on this machine:
+# pragma: allowlist secret
+#     age-keygen -o ~/.config/sops/age/keys.txt   # creates the private key
+#     age-keygen -y ~/.config/sops/age/keys.txt   # prints the public key
+#
+# Paste the printed `age1...` string in place of the placeholder.
+# See docs/secrets-management.md for the full walkthrough.
+
+keys:
+  # pragma: allowlist secret
+  # PLACEHOLDER — overwrite with your real age public key.
+  - &admin age1REPLACEME_run_age_keygen_y_then_paste_the_real_key_here
+
+creation_rules:
+  - path_regex: secrets/.*\.yaml$
+    key_groups:
+      - age:
+          - *admin