fix(fleet): download ledger artifacts per-subdir so multi-job ledgers do not clobber

.github/workflows/fleet-reconcile.yaml

@@ -121,18 +121,20 @@ jobs:
           go-version-file: go.mod
           cache: true
 
-      # Merge every per-job ledger artifact into one JSONL file. merge-multiple
-      # concatenates same-named files across artifacts; the glob captures every
-      # registering job's upload. Missing artifacts are tolerated (a suite that
-      # gated nothing, or one using the in-workspace ledger path instead).
+      # Download every per-job ledger artifact, each into its own subdirectory.
+      # Do NOT use merge-multiple: the per-job files all share the basename
+      # cascade-run-ledger.jsonl, so flattening them into one directory would
+      # overwrite each other and only one job's ledger would survive. With
+      # per-artifact subdirectories the assemble step's recursive find then
+      # concatenates one file per job. Missing artifacts are tolerated (a suite
+      # that gated nothing, or one using the in-workspace ledger path instead).
       - name: Download run-ledger artifacts
         id: ledger
         continue-on-error: true
         uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5.0.0
         with:
           pattern: ${{ inputs.ledger-artifact }}
           path: ${{ runner.temp }}/ledger-artifacts
-          merge-multiple: true
 
       - name: Assemble the ledger
         id: assemble