mcp-shell: Secure Mode Allowlist Bypass via Default `/bin/bash` Executable

mcp-shell is an MCP server for running shell commands securely, auditably, and on demand. Prior to 0.6.0, the default Docker security.yaml includes /bin/bash in allowed_executables, while security.go validates only the first token and checkBlockedPatternsAndCommands does not reject the shell command-mode flag -c. A caller of the shell_exec MCP tool can provide the command argument `/bin/bash -c <arbitrary-command>`, which passes validation and reaches executor.go, where parseCommand and exec.CommandContext execute the arbitrary command as mcpuser outside the intended allowlist. This issue is fixed in version 0.6.0.