Add a pinned secret-scanning workflow

Scans the full git history on push, pull request and weekly, so a credential
that was committed and later removed is still detected. The scanner archive is
pinned by version and verified by SHA-256 before it runs, and matched values
are redacted so they never reach the CI log.

The accompanying allowlist covers two redaction test fixtures that assert
secret-shaped input is masked. Each entry is pinned to an exact commit, file,
rule and line, so a real secret added to those files later is still reported.

.gitleaksignore

@@ -0,0 +1,13 @@
+# Allowlisted Gitleaks fingerprints.
+#
+# Both entries are deliberate test fixtures for this project's own redaction
+# function, not credentials: each asserts that a secret-shaped input is masked.
+# They are history-only -- the current tests assemble those strings at runtime --
+# and a historical commit cannot be cleaned without rewriting history, which is
+# not warranted for fixture data.
+#
+# Each entry is pinned to an exact commit, file, rule and line rather than
+# ignoring the tests/ directory, so a real secret committed to the same files
+# later is still reported.
+a9e828706529bab38759f9a8bea4aafc721853aa:tests/test_redact.py:generic-api-key:30
+a9e828706529bab38759f9a8bea4aafc721853aa:tests/test_judge.py:generic-api-key:71