Considering debug mode for step-by-step DAG execution - what do you think?

This is a proposal for adding debug capabilities to Dagu. Before we build this, we'd love to hear:
- Would this solve your debugging pain points?
- Which features would you use most?
- What's missing from this proposal?

Please feel free to share your thoughts in the comments!

### Problem
When DAGs fail, developers only have logs. They need to pause execution and inspect what's happening in real-time.

### Solution
Add debugging capabilities: breakpoints, step-through execution, and variable inspection.

**Execution Control:**
- Set breakpoints on steps
- Pause and resume execution
- Execute one step at a time

**State Inspection:**
- View all variables and their values
- Inspect step outputs

### Roadmap (draft)
**Ph1**
- Pause/resume at named steps
- Step-through execution
- CLI support

**Ph2**
- Variable watch lists
- View current variables
- Web UI debugger

**Ph3**
- Modify variables during debug

The issue names no files, tests, or entry points. Start by resolving which Phase 1 behavior and interface are required, then identify the CLI and DAG execution components to change; done should mean an agreed scope and acceptance criteria before implementation.

Written by the indexing model from the issue text.