feat(agent): add progress checkpointing for long-running tasks

Implement comprehensive progress tracking in ExecutionContext for long-
running agent tasks. This enables:

- Periodic progress checkpoints with iteration tracking
- Milestone detection (25%, 50%, 75%, 90%, 100% completion)
- Action success/failure statistics with success rate
- Token usage and API call tracking
- Disk persistence for checkpoint recovery
- Resumption strategies (continue, retry, skip, rebuild)

Key additions:
- ProgressData struct with completion estimates and metrics
- ProgressMilestone enum for milestone detection
- ProgressRecoveryInfo for guiding resumption
- 15+ progress tracking methods on ExecutionContext
- 5 comprehensive unit tests

Closes fluent_cli-8e3

Author: njfio