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>

secrets/README.md

@@ -0,0 +1,39 @@
+# secrets/
+
+Encrypted YAML files in this directory are decrypted at NixOS
+activation time and exposed under `/run/secrets/<name>` for any
+service that declares `sops.secrets.<name>` to consume.
+
+Recipients are declared in `../.sops.yaml`. The matching age
+private key lives at `~/.config/sops/age/keys.txt` on the host
+machine (see `modules/secrets.nix`).
+
+## Workflow
+
+```sh
+# First-time: create + encrypt this host's secrets file
+sops secrets/<hostName>.yaml
+# sops auto-encrypts on save using recipients from .sops.yaml
+
+# Later edits go through sops (auto-decrypts, re-encrypts on save)
+sops secrets/<hostName>.yaml
+```
+
+See [`../docs/secrets-management.md`](../docs/secrets-management.md)
+for the full walkthrough — generating the age key, adding a recipient,
+declaring a secret in NixOS, and rotating keys.
+
+## What goes here
+
+One YAML file per host, named after the host. Inside each file, a
+flat or nested map of secret names → values:
+
+```yaml
+# secrets/<hostName>.yaml — encrypted in place
+lnbits-admin-key: changeme-real-key-goes-here
+postgres:
+  lnbits-password: changeme-real-password-goes-here
+```
+
+NixOS modules reference these by name via `sops.secrets.<name>`
+and read the runtime path via `config.sops.secrets.<name>.path`.