feat: Matrix task and DAG execution

Proposal to implement matrix execution on both task and DAG level.

Dependencies:

- https://github.com/dagu-org/dagu/issues/786#issuecomment-2581684809
- #738

Reference to other implementations:

- [Running variations of jobs in a workflow - GitHub Docs](https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/running-variations-of-jobs-in-a-workflow)
- [rfcs/029-across-step/proposal.md at master · concourse/rfcs](https://github.com/concourse/rfcs/blob/master/029-across-step/proposal.md)

Start by reading the dependency discussion in issue #786 and issue #738, then compare the GitHub Actions matrix documentation with Concourse's across-step RFC. Define the expected matrix behavior at both task and DAG level before locating the workflow execution entry points; done means both scopes support the agreed variations consistently.

Written by the indexing model from the issue text.