Skip non-frontend stacks on frontend-only PRs

[Yicong-Huang]

Task Summary

Skip non-frontend stacks (scala, python, agent-service) when a PR is labelled as touching only the frontend (and optionally docs / dev). Mirror of the existing "skip frontend if no frontend label" rule introduced in #4622.

Context

Today's precheck decision table from #4622:

PR labels	frontend	scala	python	agent-service
only docs and/or dev	skip	skip	skip	skip
no frontend label	skip	run	run	run
includes frontend (or any non-skip label)	run	run	run	run

The third row is over-broad: a pure frontend change (only the frontend label, plus optionally docs / dev) still triggers the full scala / python / agent-service matrix. That is ~30 minutes of CI for code paths that are guaranteed unaffected.

Proposed rule

If the PR's labels are a subset of {frontend, docs, dev} and frontend is present, skip scala, python, and agent-service; only run frontend. Combined table:

PR labels	frontend	scala	python	agent-service
only docs and/or dev	skip	skip	skip	skip
frontend only (with any of docs, dev)	run	skip	skip	skip
no frontend label	skip	run	run	run
any other combination	run	run	run	run

Priority

P2 – Medium

Task Type

• Code Implementation
• DevOps / Deployment