docs: run the checks CI runs, and what a green one does not prove

CONTRIBUTING.md

@@ -9,7 +9,8 @@ Install the development dependencies and run the suite from the repository root:
 ```bash
 pip install -e ".[dev]"
 pytest
-ruff check src tests
+ruff check src tests scripts
+python tools/check_dashes.py
 mypy src/agentrust_trace
 ```
 
@@ -18,6 +19,16 @@ wheel installed elsewhere in the environment must not shadow the code under
 test; a regression test fails with the resolved import path if that guarantee is
 lost.
 
+A check turning green is not a review of the edit that turned it green. Each of
+these was built to see one thing, and none of them sees what your repair
+introduced. `tools/check_dashes.py` bans four characters and prints the form to
+use instead, and it cannot tell whether you used that form. Take an en dash out
+of a range, put a bare `to` in its place, and you have `1to3` where `1 to 3` was
+meant, with the check passing: a ban on a character cannot see the shape of the
+replacement. Read the changed line in the shape a reader meets it, rendered
+rather than as source, and run something that could have caught the new mistake.
+The check you just fixed is not that something.
+
 ## Using AI to contribute
 
 Use agents. A lot of this was built with them and saying otherwise would be dishonest.