feat: add OAuth token authentication support

[amondnet]

• Made claude-api-key input optional
• Introduced claude-code-oauth-token input for alternative authentication
• Updated environment variable echo for CLAUDE_CODE_OAUTH_TOKEN

[zhuravel]

This didn't work for me.

Content of claudecode-results.json:

{"error": "Claude Code not available: ANTHROPIC_API_KEY environment variable is not set"}

ANTHROPIC_API_KEY is checked here:

claude-code-security-review/claudecode/github_action_audit.py

Line 328 in c51ac4f

return False, "ANTHROPIC_API_KEY environment variable is not set"

claude-code-security-review/claudecode/github_action_audit.py

Lines 566 to 570 in c51ac4f

	# Validate Claude Code is available
	claude_ok, claude_error = claude_runner.validate_claude_available()
	if not claude_ok:
	print(json.dumps({'error': f'Claude Code not available: {claude_error}'}))
	sys.exit(EXIT_GENERAL_ERROR)