docs: update for sops-to-openbao migration completion

Update CLAUDE.md and README.md to reflect that secrets are now managed
by OpenBao, with sops only remaining for ca. Update migration plans
with sops cleanup checklist and auth01 decommission.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/completed/sops-to-openbao-migration.md

@@ -1,6 +1,22 @@
 # Sops to OpenBao Secrets Migration Plan
 
-## Status: In Progress
+## Status: Complete (except ca, deferred)
+
+## Remaining sops cleanup
+
+The `sops-nix` flake input, `system/sops.nix`, `.sops.yaml`, and `secrets/` directory are
+still present because `ca` still uses sops for its step-ca secrets (5 secrets in
+`services/ca/default.nix`). The `services/authelia/` and `services/lldap/` modules also
+reference sops but are only used by auth01 (decommissioned).
+
+Once `ca` is migrated to OpenBao PKI (Phase 4c in host-migration-to-opentofu.md), remove:
+- `sops-nix` input from `flake.nix`
+- `sops-nix.nixosModules.sops` from all host module lists in `flake.nix`
+- `inherit sops-nix` from all specialArgs in `flake.nix`
+- `system/sops.nix` and its import in `system/default.nix`
+- `.sops.yaml`
+- `secrets/` directory
+- All `sops.secrets.*` declarations in `services/ca/`, `services/authelia/`, `services/lldap/`
 
 ## Overview