naros: fix agent Dockerfile jq continuation; add corpus overhead harness

The manifest-stamping RUN's multi-line jq filter lacked Dockerfile line
continuations (newlines inside shell quotes do not continue an instruction),
so the first real docker build died with 'unknown instruction: |'. Join the
filter with backslash continuations — the parser strips them, jq receives one
line.

shell/corpus/overhead.py is the M3 <3% wall-clock gate harness: alternating
bash/nash corpus rounds (observation spooling on for nash), median totals.

Co-Authored-By: Claude Fable 5 <[email protected]>

images/agent/Dockerfile

@@ -180,10 +180,10 @@ RUN set -eu; \
     pw_v="$(playwright --version | awk '{print $2}')"; \
     jq --arg node "$node_v" --arg python "$python_v" --arg rust "$rust_v" \
        --arg go "$go_v" --arg mise "$mise_v" --arg gcc "$gcc_v" --arg playwright "$pw_v" \
-       '.tier = "agent"
-        | .toolchains = {node: $node, python: $python, rust: $rust, go: $go, mise: $mise, gcc: $gcc}
-        | .caches = {npm: "/opt/cache/npm", pip: "/opt/cache/pip", uv: "/opt/cache/uv",
-                     cargo: "/opt/cache/cargo", gomod: "/opt/cache/gomod"}
+       '.tier = "agent" \
+        | .toolchains = {node: $node, python: $python, rust: $rust, go: $go, mise: $mise, gcc: $gcc} \
+        | .caches = {npm: "/opt/cache/npm", pip: "/opt/cache/pip", uv: "/opt/cache/uv", \
+                     cargo: "/opt/cache/cargo", gomod: "/opt/cache/gomod"} \
         | .playwright = {version: $playwright, browsers: "/opt/playwright-browsers", chromium: true}' \
        /etc/naros/manifest.json > /etc/naros/manifest.json.new; \
     mv /etc/naros/manifest.json.new /etc/naros/manifest.json; \