ci: keep Prediction Markets main runs from cancelling each other

[proerror77]

Change contract

Prediction Markets CI keeps every main push run alive while still cancelling superseded runs on PR and other non-main refs.

Evidence

Five main runs were cancelled by later pushes on 2026-07-29. The broadened Aliyun main trigger in #613 would make the same gap easier to reproduce: a deployment-only replacement run does not necessarily rerun source-selected image lanes from the cancelled commit.

Acceptance

• cancel-in-progress is false only for refs/heads/main
• PR and other non-main runs still cancel superseded executions
• A focused workflow contract test pins the exact expression
• Existing scope selection is unchanged

Dependencies

None. This must merge before #613 resumes because both PRs modify .github/workflows/ploy-ci.yml and its contract test.

Out of scope

• Changing Monorepo CI concurrency, which already protects main
• Cache key or sccache changes
• Aliyun scope routing itself
• ACR publication or deployment

Rollout/rollback

CI scheduling only. Rollback is revert, which restores the documented main-run cancellation risk.