fix: isolate test-mode runtime artifacts

[fazxes]

Summary

• isolate nightshift test runtime artifacts under a temp-root runtime directory so evaluation clones stay clean
• teach artifact parsing to read isolated test-mode logs/state while keeping normal run mode behavior unchanged
• add regression coverage plus tracker/changelog/handoff updates for task Eval score gate: block normal tasks until eval score reaches 80/100 #100

Test plan

• make check
• bash scripts/validate-docs.sh
• bash scripts/validate-tasks.sh
• smoke-tested python3 -m nightshift test --agent codex --cycles 0 --cycle-minutes 1 --repo-dir <tmp repo> and verified the repo stayed clean