fix(e2e/docker): remove COPY of gitignored plugin README

The OpenCode E2E Docker image was copying packages/plugin/README.md
into /test/mc-opencode/README.md, but that file is gitignored — it's
regenerated locally by scripts/release.sh from the root README at
publish time. CI doesn't have the file on disk, so docker build
failed every time it ran on master:

  ERROR: failed to compute cache key: failed to calculate checksum
  of ref ... "/packages/plugin/README.md": not found

The COPY destination was also never read inside the image — pure
dead step. Removed it and left a comment explaining why future
edits shouldn't add it back. Verified locally:

  docker buildx build ... → DONE 6.1s
  docker run --rm ... → PASS: 10  FAIL: 0

Pi side was unaffected (Pi's README is committed) and Pi E2E was
already passing in CI.

Author: ualtinok

tests/docker/Dockerfile.opencode

@@ -61,7 +61,10 @@ RUN git config --global user.email "test@cortexkit.local" && \
 # already produced packages/plugin/dist/.
 COPY packages/plugin/dist/ /test/mc-opencode/dist/
 COPY packages/plugin/package.json /test/mc-opencode/package.json
-COPY packages/plugin/README.md /test/mc-opencode/README.md
+# README is intentionally NOT copied — it's gitignored locally (regenerated
+# by scripts/release.sh from root README at publish time) and unused inside
+# the image. Pinning a COPY here would break CI on master where the file
+# doesn't exist on disk.
 
 # Test scripts and aimock fixtures.
 COPY tests/docker/test-opencode-e2e.sh /test/test-opencode-e2e.sh