Add task outcome evidence gate

[baskduf]

Summary

• Add a trigger-based task outcome evidence decision gate for substantial harness work.
• Document the operational evidence loop and record the decision in ADR 0006.
• Strengthen check_effectiveness_plan.py so included task outcomes must carry comparable evidence fields.
• Add dogfood task outcome evidence for this harness-maintenance change.

Checks Run

python3 -m unittest discover -s tests
python3 -m py_compile scripts/apply_harness.py scripts/check_docs_drift.py scripts/check_structure.py scripts/check_encoding_hygiene.py scripts/check_effectiveness_plan.py scripts/check_failure_memory.py scripts/check_decision_memory.py scripts/harness_doctor.py
python3 scripts/check_docs_drift.py
python3 scripts/check_structure.py
python3 scripts/check_encoding_hygiene.py
python3 scripts/check_effectiveness_plan.py
python3 scripts/check_failure_memory.py
python3 scripts/check_decision_memory.py
python3 scripts/harness_doctor.py --target .
python3 scripts/check_effectiveness_plan.py --require-report
python3 -m unittest tests.test_check_effectiveness_plan

Additional manual fixture check: verified valid/incomplete task outcome records against the checker in temporary target directories, including the installer-generated generic checker copy.

Review

• Read-only sub-agent review found an empty inline expected_boundary: [] validation gap.
• Fixed with placeholder handling and a regression test.
• Re-review result: No findings.

Notes

• FastAPI E2E remains opt-in and was not run because this change does not touch FastAPI profile behavior.
• Harness Doctor remains 98/100; the existing known-limitations README heuristic gap is unchanged.