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/default.nix

@@ -0,0 +1,8 @@
+# Shared agent module exports
+# Imports all shared modules for the coding.agents namespace.
+{
+  imports = [
+    ./git-identity.nix
+    ./shared-options.nix
+  ];
+}

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";
+    };
+  };
+}

modules/home-manager/coding/agents/shared/shared-options.nix

@@ -0,0 +1,77 @@
+# Shared option definitions for agent modules.
+# Prevents copy-pasting the externalSkills submodule across opencode/claude-code/pi.
+{lib}: let
+  inherit (lib) mkOption mkEnableOption types literalExpression;
+in {
+  # Common agentsInput option used by all agent modules.
+  mkAgentsInputOption = description:
+    mkOption {
+      type = types.nullOr types.anything;
+      default = null;
+      inherit description;
+    };
+
+  # Common modelOverrides option.
+  mkModelOverridesOption = mkOption {
+    type = types.attrsOf types.str;
+    default = {};
+    description = ''
+      Per-agent model overrides. Maps agent slug to model string.
+      Example: { chiron = "anthropic/claude-sonnet-4"; }
+    '';
+    example = literalExpression ''
+      {
+        chiron = "anthropic/claude-sonnet-4";
+        "chiron-forge" = "anthropic/claude-sonnet-4";
+      }
+    '';
+  };
+
+  # External skills submodule — used by opencode, claude-code, and pi modules.
+  externalSkillsOption = mkOption {
+    type = types.listOf (types.submodule {
+      options = {
+        src = mkOption {
+          type = types.anything;
+          description = "Flake input pointing to a skills repository root.";
+        };
+        skillsDir = mkOption {
+          type = types.str;
+          default = "skills";
+          description = ''
+            Subdirectory inside src that contains skill folders.
+          '';
+        };
+        selectSkills = mkOption {
+          type = types.nullOr (types.listOf types.str);
+          default = null;
+          description = ''
+            List of skill names to cherry-pick from this source.
+            null means include every skill found in skillsDir.
+          '';
+        };
+      };
+    });
+    default = [];
+    description = ''
+      External skill sources passed to mkOpencodeSkills.
+      Each entry maps directly to an element of the externalSkills
+      list accepted by the AGENTS flake's lib.mkOpencodeSkills.
+    '';
+    example = literalExpression ''
+      [
+        { src = inputs.skills-anthropic; selectSkills = [ "claude-api" ]; }
+        { src = inputs.basecamp; }
+      ]
+    '';
+  };
+
+  # Helper to map externalSkills from module config to mkOpencodeSkills format.
+  mapExternalSkills = cfgEntries:
+    map (
+      entry:
+        {inherit (entry) src skillsDir;}
+        // lib.optionalAttrs (entry.selectSkills != null) {inherit (entry) selectSkills;}
+    )
+    cfgEntries;
+}