security: allowlist the known test fixture gitleaks flags (infra#317)

The estate-wide secret scan (infra#316) flags a fixture value in this repo
as a credential. Verified by reading the surrounding code that it is not
one, then scoped an allowlist to it.

Scoped by BOTH path and pattern (condition = "AND"), never path alone: a
path-only allowlist would blind this file to a future REAL secret, which is
how a live token hid inside a file whose name read as safe (infra#256).
Allowlist by shape, not by filename.

Adds .gitleaks.toml where there was none, so this repo is no longer scanned
with bare defaults.

.gitleaks.toml

@@ -0,0 +1,26 @@
+# gitleaks config for the jepa-fx-risk repo (infra#317).
+#
+# Runs gitleaks' full default ruleset and allowlists one non-credential
+# constant, scoped by BOTH path and pattern (condition = "AND") so a different
+# secret in the same files still fires. Allowlist by shape, not by filename.
+
+title = "jepa-fx-risk gitleaks config"
+
+[extend]
+useDefault = true
+
+# METRIC_KEY is the metrics-pipeline identifier
+# ("VaR_breach_rate_99_oos_regime_cond") that train.py's VaR eval block writes
+# into metrics.json -- documented in var_breach.py's own comment. It trips
+# generic-api-key purely because the variable name ends in _KEY and the value
+# is long and mixed-case enough to pass the entropy heuristic. It is not a
+# credential and has no secret analogue.
+[[allowlists]]
+description = "infra#317 — METRIC_KEY is a metrics identifier, not a credential"
+condition = "AND"
+regexTarget = "line"
+paths = [
+  '''(^|/)scripts/var_breach\.py$''',
+  '''(^|/)tests/test_var_metric\.py$''',
+]
+regexes = ['''METRIC_KEY''']