fix(modules-1,15,17): onboarding step, make M15 gate actually catch the plant, M17 .env override

- M1: add a no-git "Get the course materials" step (download+unzip; clone noted
  as Module 8) so Part A's paths resolve without assuming git. URL flagged
  Verify-before-publish (swap to public host before publishing).
- M15: security gate was failing OPEN on python3-only systems (bare `python`)
  and missing the UNTRACKED config.py, so the planted secret passed green. Now
  guards python3, fails CLOSED on any non-clean exit, and stages files so the
  planted SYNC_API_KEY + typosquat dep are actually caught.
- M15: correct the false "Bandit flags the API key" claim (B105-107 need
  password-named ids); add an honest MD5 (B324) flaw so the SAST demo fires.
  Planted secret/deps preserved.
- M17: require the .env loader to use setdefault so Part D's override demo works;
  explain precedence. Hardcoded "before" anti-pattern left intact.

Closes #6
Closes #17
Closes #18
Closes #19
Closes #29

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TfzV5QvtPDz8LJS3Pu5VLT

modules/15-security-scanning/lab/security-scan.sh

@@ -14,6 +14,13 @@
 
 set -u  # treat unset vars as errors; we manage exit codes explicitly below.
 
+# A security gate must fail CLOSED. If the interpreter the secret gate needs isn't here, abort with a
+# non-zero exit rather than sailing past the check and reporting a false "passed".
+command -v python3 >/dev/null 2>&1 || {
+  echo ">> python3 is required for the secret gate but was not found. Aborting." >&2
+  exit 2
+}
+
 status=0
 
 echo "=== Gate 1: SCA / dependency scan (pip-audit) ==="
@@ -28,16 +35,33 @@ fi
 
 echo
 echo "=== Gate 2: secret scan (detect-secrets) ==="
-# detect-secrets prints a JSON report of any secrets it finds. We treat a non-empty results set as a
-# failure. `python -c` keeps this portable (no jq dependency).
+# detect-secrets prints a JSON report of any secrets it finds. NOTE: with no path it scans the files
+# git TRACKS, so stage the starter files (`git add`) before running this, or an untracked file is
+# invisible to the gate. We parse the JSON with `python3` (no jq dependency) and fail CLOSED: the
+# parser returns 0=secrets found, 1=clean, anything else=couldn't tell — and "couldn't tell" must
+# count as a failure, never a silent pass.
 report="$(detect-secrets scan)"
-if printf '%s' "$report" | python -c 'import sys, json; sys.exit(0 if json.load(sys.stdin).get("results") else 1)'; then
-  echo "$report"
-  echo ">> SECRET gate FAILED: a credential was detected in the tree. See report above." >&2
-  status=1
-else
-  echo "no secrets detected."
-fi
+printf '%s' "$report" | python3 -c 'import sys, json
+try:
+    found = bool(json.load(sys.stdin).get("results"))
+except Exception:
+    sys.exit(2)
+sys.exit(0 if found else 1)'
+secret_rc=$?
+case "$secret_rc" in
+  0)
+    echo "$report"
+    echo ">> SECRET gate FAILED: a credential was detected in the tree. See report above." >&2
+    status=1
+    ;;
+  1)
+    echo "no secrets detected."
+    ;;
+  *)
+    echo ">> SECRET gate ERROR: could not parse the scan output (exit $secret_rc). Failing closed." >&2
+    status=1
+    ;;
+esac
 
 echo
 if [ "$status" -ne 0 ]; then