Update documentation with compaction probe details.

Related to: PR #2047 (compaction probe validation #1609)
Phase: 3 (post-MVP integration)

Update docs/src/concepts/context-management.md with:

1. Section: Post-Compression Validation

   • Explain probe purpose (detect lossy summarization)
   • Describe three-tier verdict system
   • Example probe output in debug dumps

2. Config Reference

   • [memory.compression.probe] section schema
   • Threshold tuning guidelines
   • When to enable/disable

3. Troubleshooting

   • How to interpret probe warnings
   • How to adjust thresholds
   • Performance considerations

4. Reference: Link to Factory.ai evaluation framework and design doc at .local/plan/compaction-probe-design.md