ci: a manifest contract warning passes the pull_request run and only fails in the merge queue #6215

This was generated by AI during work-loop.

Context

PR #6209 passed every pull_request check, then failed the merge queue twice on scripts/validate-plugin-contracts.test.sh's zero-warnings assertion: a harness-ops userConfig description was 308 characters, over the 300 budget. On the pull_request run, check-plugins printed that warning (run 37191738165) but exited 0, and the selective test-bash plan ran only sync-run.test.sh, so nothing failed until the merge group ran the full shard set. Each such miss costs a merge-queue round trip and blocks the queue behind it.

Proposed work

Make a manifest-contract warning fail on pull_request when the PR touches a plugin manifest. Options: let check-plugins exit non-zero on a warning, or have the test-bash planner always select validate-plugin-contracts.test.sh when a plugin.json changes. Decide which; do not weaken the merge-group assertion.

Acceptance criteria

• A PR that pushes a userConfig description over budget fails on its pull_request run.
• A PR touching no manifest is not slowed or failed by the change.

References

• PR feat(harness-ops): enable the plugins sync ask picks the CLI installed disabled #6209; runs 37191738165 (pull_request), 37191942901 and 37192914473 (merge_group)

Metadata

Filed by the work-loop lane (instance melo-worker-1) per the self-observation contract; not self-triaged.