feat: add agenix for secrets management

secrets/REAME.md

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+# secrets management
+
+This directory contains my secret files, encrypt by agenix:
+
+- my wireguard configuration files, which is used by `wg-quick`
+- github token, used by nix flakes to query and downloads flakes from github
+  - without this, you may reach out github api rate limit.
+- ssh key pairs for my homelab and other servers
+- ...
+
+## Add or Update Secrets
+
+This job is done by `agenix` CLI tool with the `./secrets.nix` file.
+
+Pretend you want to add a new secret file `xxx.age`, then:
+
+1. `cd` to this directory
+1. edit `secrets.nix`, add a new entry for `xxx.age`, which defines the
+   encryption keys and the secret file path, e.g.
+  ```nix
+  # This file is not imported into your NixOS configuration. It is only used for the agenix CLI.
+  # agenix use the public keys defined in this file to encrypt the secrets.
+  # and users can decrypt the secrets by any of the corresponding private keys.
+
+  let
+    # get user's ssh public key by command:
+    #     cat ~/.ssh/id_ed25519.pub
+    # if you do not have one, you can generate it by command:
+    #     ssh-keygen -t ed25519
+    ryan = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJx3Sk20pLL1b2PPKZey2oTyioODrErq83xG78YpFBoj";
+    users = [ ryan ];
+
+    # get system's ssh public key by command:
+    #    cat /etc/ssh/ssh_host_ed25519_key.pub
+    msi-rtx4090 = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICGeXNCazqiqxn8TmbCRjA+pLWrxwenn+CFhizBMP6en root@msi-rtx4090";
+    systems = [ msi-rtx4090 ];
+  in
+  {
+    "./encrypt/xxx.age".publicKeys = users ++ systems;
+  }
+  ```
+2. create and edit the secret file `xxx.age` interactively by command:
+  ```shell
+  agenix -e ./encrypt/xxx.age
+  ```
+3. or you can also encrypt an existing file to `xxx.age` by command:
+  ```shell
+  agenix -e ./encrypt/xxx.age < /path/to/xxx
+  ```
+
+
+## Deploy Secrets
+
+This job is done by `nixos-rebuild` with the `./default.nix` file.
+
+An nixos module exmaple(need to set agenix as flake inputs first...):
+
+```nix
+{ config, pkgs, agenix, ... }:
+
+{
+  imports = [
+     agenix.nixosModules.default
+  ];
+
+  environment.systemPackages = [
+    agenix.packages."${pkgs.system}".default 
+  ];
+
+  age.secrets."xxx" = {
+    # wether secrets are symlinked to age.secrets.<name>.path
+    symlink = true;
+    # target path for decrypted file
+    path = "/etc/xxx/";
+    # encrypted file path
+    file = ./encrypt/xxx.age;
+    mode = "0400";
+    owner = "root";
+    group = "root";
+  };
+}
+```
+
+`nixos-rebuild` will decrypt the secrets using the private keys defined by argument `age.identityPaths`,
+And then symlink the secrets to the path defined by argument `age.secrets.<name>.path`, it defaults to `/etc/secrets`.
+
+NOTE: `age.identityPaths` it defaults to `~/.ssh/id_ed25519` and `~/.ssh/id_rsa`, so you should put your decrypt keys there. if you're deploying to the same machine as you're encrypting from, it should work out of the box.