fix(sleep): add optional per-task regression gate

[Langning Zhang (Boulea7)]

Summary

• add a default-off gate_no_regression option that rejects skill, memory, or final candidates when any validation task gets worse
• record observed per-task score changes for intermediate trials and the final replay in the human and machine-readable reports
• treat missing or non-finite task results as regressions when the option is enabled, while keeping persisted JSON standard and redacted

Testing

• python3.12 -m pytest -q (953 passed, 6 skipped, 130 subtests)
• python3.12 -m mkdocs build --strict
• ruff check on the changed Python files
• python3.12 -m compileall -q skillopt_sleep tests/test_gate_no_regression.py

Closes #174