docs: sops-nix secrets workflow

Operator runbook for adding a secret, adding a new host, and rotating
keys, plus a pointer from CLAUDE.md's architecture overview.

docs/secrets.md

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+# Secrets (sops-nix)
+
+Secrets are encrypted with [sops](https://github.com/getsops/sops) and decrypted
+at activation by [sops-nix](https://github.com/Mic92/sops-nix). Each host decrypts
+with its own SSH host key (converted to an age key in-memory — no separate host
+private key is ever stored on disk).
+
+Tooling (`sops`, `age`, `ssh-to-age`) is available via `nix develop` in this repo,
+and installed system-wide on every machine via `machines/core/sops.nix`.
+
+## Adding a secret to an existing host
+
+```
+nix develop -c sops secrets/<host>.yaml
+```
+
+This opens `$EDITOR` on the decrypted content. Add your key, save, quit — sops
+re-encrypts on write. Then declare it in `machines/<host>/secrets.nix`:
+
+```nix
+sops.secrets.my-secret = {
+  owner = "someuser";   # defaults to root
+  group = "someuser";
+  mode = "0400";
+  restartUnits = [ "foo.service" ];  # if a running service needs to pick it up
+};
+```
+
+Consume it in the owning module as `config.sops.secrets.my-secret.path`
+(resolves to `/run/secrets/my-secret`) — never inline the decrypted value into
+a Nix string, since that copies it into the world-readable `/nix/store`.
+
+Then:
+
+```
+nix build '.#nixosConfigurations.<host>.config.system.build.toplevel'
+sudo nixos-rebuild test --flake '.#<host>'
+# verify, then:
+sudo nixos-rebuild switch --flake '.#<host>'
+```
+
+**The secrets file must be `git add`ed before `nix build`** — untracked files
+are invisible to flake evaluation, and `sops.validateSopsFiles` needs it in the
+Nix store.
+
+## Adding a new host
+
+1. On the new host: `ssh-to-age < /etc/ssh/ssh_host_ed25519_key.pub` to get its
+   age public key.
+2. In `.sops.yaml`: add a `&host_<name>` anchor with that key, and a
+   `creation_rules` entry for `secrets/<name>.yaml` including both the new
+   host key and `*admin_finn`.
+3. Create `machines/<name>/secrets.nix`, add it to that machine's
+   `configuration.nix` imports.
+4. If the host needs to read an existing shared file, run
+   `sops updatekeys secrets/<file>.yaml` after step 2 so it's re-encrypted to
+   include the new recipient.
+
+## Rotating keys
+
+```
+# after editing .sops.yaml (new/removed recipient):
+nix develop -c sops updatekeys secrets/<file>.yaml
+```
+
+for every file the change affects. `updatekeys` requires you to already be
+able to decrypt the file — this is why `*admin_finn` is a recipient on every
+rule, as the standing fallback.
+
+Reinstalling a host regenerates its SSH host key, which changes its age
+identity. Any file previously encrypted to that host must be `updatekeys`'d
+with the new key, or the host's next activation fails to decrypt.
+
+## Where private keys live
+
+- **Operator** (you): `~/.config/sops/age/keys.txt` — sops' default lookup
+  path, so `SOPS_AGE_KEY_FILE` never needs to be set. Never committed. Back
+  this up out-of-band (password manager, offline copy) — if it's lost, only
+  the surviving host keys can still decrypt their own files, and any file that
+  was only ever readable by an operator-only rule becomes permanently
+  unreadable.
+- **Hosts**: `/etc/ssh/ssh_host_ed25519_key` — already managed by NixOS,
+  converted to age in-memory at activation time. Nothing extra to back up.
+
+## Layout
+
+```
+.sops.yaml              # age recipients + creation rules (which keys can decrypt which files)
+secrets/<host>.yaml      # per-host encrypted secrets (default sopsFile for that host)
+machines/core/sops.nix   # applied to every machine: decryption config, sops/age/ssh-to-age on PATH
+machines/<host>/secrets.nix   # per-host: which secrets exist, owner/group/mode
+```