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

docs/security.md

@@ -38,7 +38,64 @@ The following permissions are requested but not yet actively used. These will en
 
 ## Commit Signing
 
-Commits made by Claude through this action are no longer automatically signed with commit signatures. To enable commit signing set `use_commit_signing: True` in the workflow(s). This ensures the authenticity and integrity of commits, providing a verifiable trail of changes made by the action.
+By default, commits made by Claude are unsigned. You can enable commit signing using one of two methods:
+
+### Option 1: GitHub API Commit Signing (use_commit_signing)
+
+This uses GitHub's API to create commits, which automatically signs them as verified from the GitHub App:
+
+```yaml
+- uses: anthropics/claude-code-action@main
+  with:
+    use_commit_signing: true
+```
+
+This is the simplest option and requires no additional setup. However, because it uses the GitHub API instead of git CLI, it cannot perform complex git operations like rebasing, cherry-picking, or interactive history manipulation.
+
+### Option 2: SSH Signing Key (ssh_signing_key)
+
+This uses an SSH key to sign commits via git CLI. Use this option when you need both signed commits AND standard git operations (rebasing, cherry-picking, etc.):
+
+```yaml
+- uses: anthropics/claude-code-action@main
+  with:
+    ssh_signing_key: ${{ secrets.SSH_SIGNING_KEY }}
+    bot_id: "YOUR_GITHUB_USER_ID"
+    bot_name: "YOUR_GITHUB_USERNAME"
+```
+
+Commits will show as verified and attributed to the GitHub account that owns the signing key.
+
+**Setup steps:**
+
+1. Generate an SSH key pair for signing:
+
+   ```bash
+   ssh-keygen -t ed25519 -f ~/.ssh/signing_key -N "" -C "commit signing key"
+   ```
+
+2. Add the **public key** to your GitHub account:
+
+   - Go to GitHub → Settings → SSH and GPG keys
+   - Click "New SSH key"
+   - Select **Key type: Signing Key** (important)
+   - Paste the contents of `~/.ssh/signing_key.pub`
+
+3. Add the **private key** to your repository secrets:
+
+   - Go to your repo → Settings → Secrets and variables → Actions
+   - Create a new secret named `SSH_SIGNING_KEY`
+   - Paste the contents of `~/.ssh/signing_key`
+
+4. Get your GitHub user ID:
+
+   ```bash
+   gh api users/YOUR_USERNAME --jq '.id'
+   ```
+
+5. Update your workflow with `bot_id` and `bot_name` matching the account where you added the signing key.
+
+**Note:** If both `ssh_signing_key` and `use_commit_signing` are provided, `ssh_signing_key` takes precedence.
 
 ## ⚠️ Authentication Protection