⚙️ FEATURE-#101: Checkpoint-based durable state (resume from failure)

[FernandoCelmer]

Description

• dotflow/abc/flow.py: Added resume parameter to Flow ABC
• dotflow/core/dotflow.py: DotFlow.__init__ accepts optional workflow_id for fixed identification across executions
• dotflow/core/workflow.py: Manager accepts resume=True; Sequential, SequentialGroup, and Background check for existing checkpoints via _has_checkpoint() before executing each task
• docs_src/checkpoint/checkpoint.py: Usage example
• docs/nav/tutorial/checkpoint.md: Tutorial page with mermaid diagrams
• mkdocs.yml: Added checkpoint to nav

Motivation and Context

If a process dies mid-workflow, all progress is lost. With checkpoints, dotflow resumes from the last completed step — no server, no database, just storage.

Closes #101

Types of changes

• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly