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

action.yml

@@ -81,6 +81,10 @@ inputs:
     description: "Enable commit signing using GitHub's commit signature verification. When false, Claude uses standard git commands"
     required: false
     default: "false"
+  ssh_signing_key:
+    description: "SSH private key for signing commits. When provided, git will be configured to use SSH signing. Takes precedence over use_commit_signing."
+    required: false
+    default: ""
   bot_id:
     description: "GitHub user ID to use for git operations (defaults to Claude's bot ID)"
     required: false
@@ -181,6 +185,7 @@ runs:
         USE_STICKY_COMMENT: ${{ inputs.use_sticky_comment }}
         DEFAULT_WORKFLOW_TOKEN: ${{ github.token }}
         USE_COMMIT_SIGNING: ${{ inputs.use_commit_signing }}
+        SSH_SIGNING_KEY: ${{ inputs.ssh_signing_key }}
         BOT_ID: ${{ inputs.bot_id }}
         BOT_NAME: ${{ inputs.bot_name }}
         TRACK_PROGRESS: ${{ inputs.track_progress }}
@@ -334,6 +339,12 @@ runs:
           echo '```' >> $GITHUB_STEP_SUMMARY
         fi
 
+    - name: Cleanup SSH signing key
+      if: always() && inputs.ssh_signing_key != ''
+      shell: bash
+      run: |
+        bun run ${GITHUB_ACTION_PATH}/src/entrypoints/cleanup-ssh-signing.ts
+
     - name: Revoke app token
       if: always() && inputs.github_token == '' && steps.prepare.outputs.skipped_due_to_workflow_validation_mismatch != 'true'
       shell: bash