feat(agents): add gitIdentity module

- Renamed shared-options.nix to shared/shared-options.nix
- Created shared/default.nix importing git-identity.nix and shared-options.nix
- Created shared/git-identity.nix with gitIdentity option set:
  - enable: Toggle for agent git identity
  - name: Git author name (default: m3ta-chiron)
  - email: Git author email (default: m3ta-chiron@agentmail.to)
  - signingKey: Optional GPG signing key path
  - sshKey: SSH private key path for git push auth
- Updated opencode.nix, pi.nix, claude-code.nix to import shared/default.nix
- Restructured modules to follow proper Nix module syntax with imports at top level

modules/home-manager/coding/agents/shared/git-identity.nix

@@ -0,0 +1,64 @@
+# Git identity module for agent commits.
+# Sets GIT_AUTHOR_*, GIT_COMMITTER_*, and GIT_SSH_COMMAND environment variables.
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}: let
+  cfg = config.coding.agents.gitIdentity;
+in {
+  options.coding.agents.gitIdentity = {
+    enable = lib.mkEnableOption ''
+      Agent Git identity for commits. When enabled, sets GIT_AUTHOR_* and
+      GIT_COMMITTER_* environment variables for consistent bot identity.
+    '';
+
+    name = lib.mkOption {
+      type = lib.types.str;
+      default = "m3ta-chiron";
+      description = "Git user name for agent commits.";
+      example = "m3ta-chiron";
+    };
+
+    email = lib.mkOption {
+      type = lib.types.str;
+      default = "m3ta-chiron@agentmail.to";
+      description = "Git email for agent commits.";
+      example = "m3ta-chiron@agentmail.to";
+    };
+
+    signingKey = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
+      default = null;
+      description = ''
+        Optional GPG signing key for verified commits.
+        Set to null to disable signing.
+      '';
+      example = "/home/user/.gnupg/sign_key.gpg";
+    };
+
+    sshKey = lib.mkOption {
+      type = lib.types.path;
+      description = ''
+        Path to SSH private key for git push authentication.
+        Use agenix-managed paths like /run/agenix/m3ta-chiron-ssh-key
+        for secure secret management.
+      '';
+      example = "/run/agenix/m3ta-chiron-ssh-key";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    home.sessionVariables = {
+      # Git author/committer identity
+      GIT_AUTHOR_NAME = cfg.name;
+      GIT_AUTHOR_EMAIL = cfg.email;
+      GIT_COMMITTER_NAME = cfg.name;
+      GIT_COMMITTER_EMAIL = cfg.email;
+
+      # SSH command for git push
+      GIT_SSH_COMMAND = "ssh -i ${cfg.sshKey} -o IdentitiesOnly=yes -o StrictHostKeyChecking=accept-new";
+    };
+  };
+}