feat(agent-repo): create scratchpad/ at task branch top level

Cross-node artifact passing within a workflow manifest — nodes each run
in their own ephemeral container/pod with no shared filesystem, so an
agent node writing e.g. a verdict to /workspace/verdict is invisible to
a downstream kind: script node in the same manifest (found live
2026-09-06, bug-verify-sonnet's verdict-tx). The git task branch is the
only channel every node in a manifest actually shares: write to
scratchpad/, commit (picked up by finalize.sh's existing git add -A),
next node clones the same branch and reads it back.

scratchpad/ is scratch, never deliverable content, and must never
survive a merge into main — a CI gate in the target repo
(agent-runtimes .gitea/workflows/build-images.yaml,
no-scratchpad-in-main) is the hard backstop.

harnesses/contexts/agent-repo/v1/init.sh

@@ -123,6 +123,35 @@ if [ -d /workspace/project/results ]; then
 fi
 mkdir -p /workspace/project/results
 
+# Ensure scratchpad/ exists at the top level (cross-node artifact passing).
+# The git branch is this project's shared data channel between manifest
+# nodes — each node runs in its own ephemeral container/pod with no shared
+# filesystem, so an agent node that needs to hand data to a downstream
+# kind: script node (or another agent node) does it by writing here and
+# committing/pushing, for the next node to clone the same branch and read
+# back. NEVER merged into main — the no-scratchpad-in-main CI gate
+# (agent-runtimes .gitea/workflows/build-images.yaml) rejects any commit to
+# main that still contains this directory.
+mkdir -p /workspace/project/scratchpad
+if [ ! -f /workspace/project/scratchpad/README.md ]; then
+    cat > /workspace/project/scratchpad/README.md << 'SCRATCHPAD_EOF'
+# scratchpad/
+
+Ephemeral, per-task working data — NOT deliverable content.
+
+- Use this directory to pass data between nodes in the same workflow
+  manifest (e.g. an agent node writes a verdict/artifact here for a
+  downstream `kind: script` node to read back off the SAME branch — the
+  git branch is the only thing nodes in a manifest share, since each runs
+  in its own ephemeral container).
+- Anything under `scratchpad/` is scratch, not review/deliverable content.
+  **Do not** reference or copy `scratchpad/` content into the actual
+  deliverable during integration/merge.
+- A CI gate rejects any commit to `main` that still contains a
+  `scratchpad/` directory — it must never survive merge.
+SCRATCHPAD_EOF
+fi
+
 # Populate .gitignore (AR-32)
 GITIGNORE=/workspace/project/.gitignore
 cat >> "$GITIGNORE" << 'GITIGNORE_EOF'