docs: plan sops secrets with two-tier age identities

Supersede ADR 0001's SSH-derived key mechanism with an admin identity held
outside the repo plus a per-host identity on each encrypted root. Decoupling
the two is what lets the SSH host keys become secrets themselves rather than
the root of trust they were.

Add the spec, the three implementing tasks, and the glossary terms the
breakdown speaks in.

.claude/CONTEXT.md

@@ -24,6 +24,20 @@ _Avoid_: loader, importer, scanner
 The rule that every Module is imported unconditionally and guards its own body with `mkIf config.modules.<path>.enable`, so a Host reads as a checklist of `enable = true` flags.
 _Avoid_: feature flag, toggle, opt-in
 
+**admin identity**:
+The age identity held only in the operator's password manager, never committed, that is a recipient of every secrets file.
+It is the recovery path for any wiped machine and the credential that authorizes registering a new host.
+_Avoid_: master key, admin key, root key
+
+**host identity**:
+The dedicated age key on one machine's encrypted root, generated there and never transmitted, that decrypts that machine's own secrets and the shared file.
+Deliberately distinct from the machine's SSH host key.
+_Avoid_: machine key, node key, host key
+
+**secrets file**:
+One sops-encrypted file in the repo, encrypted to the admin identity plus whichever hosts may read it. Either shared across every host or specific to one.
+_Avoid_: vault, secret store, keyring
+
 **unstable overlay**:
 The overlay exposing `nixpkgs-unstable` packages as `unstable.<name>`, used to pull an individual package fresher than the `nixos-unstable` base.
 _Avoid_: bleeding-edge, latest