docs: add ASCII architecture diagrams for completion detection and loop prevention (#7)

* feat: add completion marker detection and loop prevention

Re-enable task completion detection that was removed during directory
standardization refactor. Add loop detection using rapidfuzz to prevent
agents from getting stuck in repetitive output cycles.

Changes:
- Add `- [x] TASK_COMPLETE` checkbox marker detection in orchestrator
- Add rapidfuzz-based loop detection (90% similarity threshold, 5-output window)
- Update SafetyGuard.reset() to clear loop detection history
- Add comprehensive tests for both features (23 new tests)
- Document loop detection in new advanced guide
- Update changelog and glossary

* docs: add ASCII architecture diagrams and holistic documentation updates

Add comprehensive ASCII diagrams using graph-easy to visualize:
- Sliding window mechanism for loop detection
- Loop detection decision flow
- SafetyGuard integration in orchestration loop
- Security defense layers
- Metrics collection flow
- Five safety mechanisms overview

Documentation alignment updates:
- Add loop-detection.md to mkdocs.yml navigation
- Update overview.md Section 3 with completion markers and loop detection
- Add 4 new FAQ entries about completion and loop detection
- Add troubleshooting sections for loop detection and completion markers
- Add completion marker example to quick-start.md

All diagrams include <details> blocks with graph-easy source for reproducibility.

Author: terrylica