feat(checkpoint): add checkpoint system for workflow recovery

[danyalwajid]

Description
Implement checkpoint-based recovery system for FlexibleGraphExecutor. This enables auto-saving workflow state after each successful step and resuming from the last checkpoint on failure, preventing loss of progress in long-running agent workflows.

Type of Change
Bug fix (non-breaking change that fixes an issue)
New feature (non-breaking change that adds functionality)
Breaking change (fix or feature that would cause existing functionality to not work as expected)
Documentation update
Refactoring (no functional changes)
Related Issues
Fixes #(issue number)

Changes Made
Add Checkpoint and CheckpointMetadata Pydantic schemas (framework/schemas/checkpoint.py)
Add CheckpointStorage for filesystem-based persistence using JSON files (framework/storage/checkpoint_storage.py)
Add CheckpointManager high-level API for checkpoint lifecycle management (framework/runtime/checkpoint.py)
Integrate checkpoint hooks into FlexibleGraphExecutor with auto-save after each successful step
Add resume_from_checkpoint parameter to execute_plan() for recovery support
Add checkpoint configuration options to ExecutorConfig (enabled, path, auto_cleanup)
Add comprehensive unit tests for checkpoint system (25 tests)
Testing
Describe the tests you ran to verify your changes:

tests/test_checkpoint.py - 25 passed in 1.83s

Tests cover:

• Checkpoint schema creation, serialization, deserialization
• CheckpointMetadata schema and status values
• CheckpointStorage: save, load, load_latest, get_metadata, update_status, cleanup
• CheckpointManager: save, load_latest, can_resume, on_execution_complete, disabled mode
• Integration: full save/recovery flow, multiple runs isolation, large memory states

Unit tests pass (cd core && pytest tests/)
Lint passes (cd core && ruff check .)
Manual testing performed
Test Results
Checklist
My code follows the project's style guidelines
I have performed a self-review of my code
I have commented my code, particularly in hard-to-understand areas
I have made corresponding changes to the documentation
My changes generate no new warnings
I have added tests that prove my fix is effective or that my feature works
New and existing unit tests pass locally with my changes
Screenshots (if applicable)
N/A - Backend feature, no UI changes.