feat: add ssh_signing_key input for SSH commit signing (#784)

* feat: add ssh_signing_key input for SSH commit signing

Add a new ssh_signing_key input that allows passing an SSH signing key
for commit signing, as an alternative to the existing use_commit_signing
(which uses GitHub API-based commits).

When ssh_signing_key is provided:
- Git is configured to use SSH signing (gpg.format=ssh, commit.gpgsign=true)
- The key is written to ~/.ssh/claude_signing_key with 0600 permissions
- Git CLI commands are used (not MCP file ops)
- The key is cleaned up in a post step for security

Behavior matrix:
| ssh_signing_key | use_commit_signing | Result |
|-----------------|-------------------|--------|
| not set         | false             | Regular git, no signing |
| not set         | true              | GitHub API (MCP), verified commits |
| set             | false             | Git CLI with SSH signing |
| set             | true              | Git CLI with SSH signing (ssh_signing_key takes precedence)

* docs: add SSH signing key documentation

- Update security.md with detailed setup instructions for both signing options
- Explain that ssh_signing_key enables full git CLI operations (rebasing, etc.)
- Add ssh_signing_key to inputs table in usage.md
- Update bot_id/bot_name descriptions to note they're needed for verified commits

* fix: address security review feedback for SSH signing

- Write SSH key atomically with mode 0o600 (fixes TOCTOU race condition)
- Create .ssh directory with mode 0o700 (SSH best practices)
- Add input validation for SSH key format
- Remove unused chmod import
- Add tests for validation logic

src/modes/agent/index.ts

@@ -4,7 +4,10 @@ import type { Mode, ModeOptions, ModeResult } from "../types";
 import type { PreparedContext } from "../../create-prompt/types";
 import { prepareMcpConfig } from "../../mcp/install-mcp-server";
 import { parseAllowedTools } from "./parse-tools";
-import { configureGitAuth } from "../../github/operations/git-config";
+import {
+  configureGitAuth,
+  setupSshSigning,
+} from "../../github/operations/git-config";
 import type { GitHubContext } from "../../github/context";
 import { isEntityContext } from "../../github/context";
 
@@ -79,7 +82,27 @@ export const agentMode: Mode = {
 
   async prepare({ context, githubToken }: ModeOptions): Promise<ModeResult> {
     // Configure git authentication for agent mode (same as tag mode)
-    if (!context.inputs.useCommitSigning) {
+    // SSH signing takes precedence if provided
+    const useSshSigning = !!context.inputs.sshSigningKey;
+    const useApiCommitSigning =
+      context.inputs.useCommitSigning && !useSshSigning;
+
+    if (useSshSigning) {
+      // Setup SSH signing for commits
+      await setupSshSigning(context.inputs.sshSigningKey);
+
+      // Still configure git auth for push operations (user/email and remote URL)
+      const user = {
+        login: context.inputs.botName,
+        id: parseInt(context.inputs.botId),
+      };
+      try {
+        await configureGitAuth(githubToken, context, user);
+      } catch (error) {
+        console.error("Failed to configure git authentication:", error);
+        // Continue anyway - git operations may still work with default config
+      }
+    } else if (!useApiCommitSigning) {
       // Use bot_id and bot_name from inputs directly
       const user = {
         login: context.inputs.botName,