Optional CI: skip irrelevant build stacks based on PR labels

[Yicong-Huang]

Task Summary

Umbrella for the "optional CI" line of work: make the Build workflow's main stacks (frontend, scala, python, agent-service) and the per-target backport matrix run only for the stacks a PR actually touches, driven by the labels applied by the Pull Request Labeler.

Motivation

Today every PR pays the full CI matrix (frontend × 3 OS, scala, python × 4 versions, agent-service × 2 OS, plus a backport matrix per release/* label). Pure frontend changes pay the scala/python tax; docs-only PRs wait on sbt; release-labelled docs PRs spawn an entire backport matrix that builds nothing relevant. The labels the Pull Request Labeler already produces are a near-perfect signal for what each PR actually needs to verify.

Future candidates (open as new sub-issues when prioritized)

• Symmetric rules for python / engine / service only PRs.
• Skipping individual matrix combinations (e.g. only running frontend on ubuntu-latest for non-frontend PRs that still touch shared infra).
• Treating ci / dependencies labels explicitly (currently they fall into the catch-all "run everything" branch).

Priority

P2 – Medium

Task Type

• Code Implementation
• DevOps / Deployment

[Yicong-Huang]

This task is considered done! We now fully execute conditional CIs. Hope this can make things faster!