feat: Support for operators to enhance pre conditions

Right now pre conditions provide use of commands, variables but not operators like and, or, not, contains, etc. We propose preconditions enhancements to include above.

This has been discussed by @yottahmd in detail:

https://github.com/dagu-org/dagu/issues/598

@kriyanshii has worked in this direction quite a bit.

Start by reading issue #598 and reviewing the existing precondition implementation; this payload names no files, tests, or entry points. Clarify the supported operator semantics and completion criteria before locating the affected code and tests.

Written by the indexing model from the issue text.