Title: Support for GitHub Copilot-created Pull Requests

Description

When GitHub Copilot creates a pull request, the claude-code-action fails with a 404 error because it tries to look up the PR author as a regular
GitHub user.

Steps to Reproduce

1. Use GitHub Copilot to create a pull request
2. Have claude-code-action configured to run on pull_request events
3. The action fails during permission checking

Error Log

Checking permissions for actor: Copilot
GET /users/Copilot - 404 Not Found

Expected Behavior

The action should recognize that Copilot is a GitHub app/bot, not a regular user, and handle it appropriately (similar to how copilot[bot] or
github-actions[bot] are handled).

Workaround

Currently skipping Copilot PRs with:
if: github.event.pull_request.user.login != 'Copilot'

Environment

- Action version: anthropics/claude-code-action@v1
- Configuration includes allowed_bots: 'copilot[bot],github-actions[bot]'