fix(ci): SAST actually scans the code + drop deprecated flaky semgrep action (#930)

Two real problems in the Static Application Security Testing job:

1. **It scanned a path that no longer exists.** `bandit -r src/` and
   `semgrep … src/` pointed at the repo-root `src/`, but the Python code
   moved to `archive/v1/src/` (64 .py files) when the runtime was rewritten
   in Rust. So the SAST scan matched nothing — a silent no-op (this is also
   why `bandit-results.sarif` was "Path does not exist" on recent runs).
   Fixed both to `archive/v1/src/`.

2. **Deprecated + redundant + flaky semgrep step.** The
   `returntocorp/semgrep-action@v1` step pulled `returntocorp/semgrep-agent:v1`
   from Docker Hub every run (intermittently timing out → red check, e.g. on
   #929) and is EOL. It was redundant: the pip `semgrep --sarif` step is what
   feeds GitHub Security; the action only pushed to the Semgrep cloud app via
   SEMGREP_APP_TOKEN. Removed it and folded its `p/docker` + `p/kubernetes`
   rulesets into the pip semgrep command, so coverage is preserved with no
   Docker pull.

The job stays `continue-on-error: true` (non-gating). YAML validated.

.github/workflows/security-scan.yml

@@ -46,7 +46,10 @@ jobs:
 
     - name: Run Bandit security scan
       run: |
-        bandit -r src/ -f sarif -o bandit-results.sarif
+        # The Python codebase lives under archive/v1/src (it moved there when
+        # the runtime was rewritten in Rust). Scanning `src/` matched nothing,
+        # so this SAST step was a silent no-op.
+        bandit -r archive/v1/src/ -f sarif -o bandit-results.sarif
       continue-on-error: true
 
     - name: Upload Bandit results to GitHub Security
@@ -57,22 +60,20 @@ jobs:
         sarif_file: bandit-results.sarif
         category: bandit
 
-    - name: Run Semgrep security scan
-      continue-on-error: true
-      uses: returntocorp/semgrep-action@v1
-      with:
-        config: >-
-          p/security-audit
-          p/secrets
-          p/python
-          p/docker
-          p/kubernetes
-      env:
-        SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
-        
-    - name: Generate Semgrep SARIF
+    # Removed the deprecated `returntocorp/semgrep-action@v1` step: it was
+    # redundant (the pip `semgrep --sarif` below is what feeds GitHub Security;
+    # the action only pushed to the Semgrep cloud app via SEMGREP_APP_TOKEN) and
+    # it pulled `returntocorp/semgrep-agent:v1` from Docker Hub on every run,
+    # which intermittently timed out and turned this check red. The pip semgrep
+    # (installed above) needs no Docker pull. The action's `p/docker` +
+    # `p/kubernetes` rulesets are folded into the command below so coverage is
+    # preserved.
+    - name: Run Semgrep + generate SARIF
       run: |
-        semgrep --config=p/security-audit --config=p/secrets --config=p/python --sarif --output=semgrep.sarif src/
+        semgrep \
+          --config=p/security-audit --config=p/secrets --config=p/python \
+          --config=p/docker --config=p/kubernetes \
+          --sarif --output=semgrep.sarif archive/v1/src/
       continue-on-error: true
 
     - name: Upload Semgrep results to GitHub Security