XY-915 Tighten rotated diagnostics retention to fourteen days

[yvette-carlisle]

Summary

• reduce rotated log and agent-evidence retention from 30 days to 14 days
• delete only rotated log/evidence files by age, preserving current log files and current events.jsonl streams
• document the 14-day diagnostics retention boundary

Validation

• cargo make fmt
• cargo test -p decodex prune_deletes_only_rotated_logs_and_agent_evidence_after_fourteen_days --all-features -- --test-threads=1
• cargo test -p decodex prune_rotates_oversized_logs_and_agent_evidence_events --all-features -- --test-threads=1
• cargo make lint-fix
• cargo make test
• git diff --check