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/pi.nix

@@ -3,14 +3,16 @@
   lib,
   pkgs,
   ...
-}: let
-  shared = import ./shared-options.nix {inherit lib;};
-in
-  with lib; let
-    cfg = config.coding.agents.pi;
+}: {
+  imports = [
+    ../shared/default.nix
+  ];
+
+  options.coding.agents.pi = let
+    shared = import ../shared/shared-options.nix {inherit lib;};
     mcpCfg = config.programs.mcp or null;
-  in {
-    options.coding.agents.pi = {
+  in
+    with lib; {
       enable = mkEnableOption "Pi agent management via canonical agent.toml definitions";
 
       mcpServers = mkOption {
@@ -202,7 +204,11 @@ in
       };
     };
 
-    config = mkIf cfg.enable (let
+  config = with lib; let
+    shared = import ../shared/shared-options.nix {inherit lib;};
+    cfg = config.coding.agents.pi;
+  in
+    mkIf cfg.enable (let
       # Build settings.json by filtering out null values recursively
       filterNulls = attrs:
         lib.filterAttrs (_: v: v != null) (
@@ -281,4 +287,4 @@ in
         agentFiles
       ];
     });
-  }
+}