test: bump coverage gate from 80%% to 90%%

Update --cov-fail-under in lint workflow + docs (AGENTS, README, ARCHITECTURE). Note: A5 closeout reported 80.10%% so this run will likely fail until additional tests are added.

README.md

@@ -232,10 +232,10 @@ to the Python kebab-case CLI and propagate the exit code.
 ```powershell
 .\.venv\Scripts\python.exe -m ruff check src tests\python
 .\.venv\Scripts\python.exe -m mypy --strict src
-.\.venv\Scripts\python.exe -m pytest tests\python -q --cov=ci_orchestrator --cov-fail-under=80
+.\.venv\Scripts\python.exe -m pytest tests\python -q --cov=ci_orchestrator --cov-fail-under=90
 ```
 
-Coverage gate is **80%** as of Phase A5. Regression tests for the
+Coverage gate is **90%**. Regression tests for the
 `AGENTS.md` "Errori frequenti da evitare" #9–#12 live in
 [tests/python/test_agents_errors.py](tests/python/test_agents_errors.py)
 and must stay green.