signals: use safety-filtered preferred labels in guidance

[aryansk]

Summary

• build preferred-label next-step text from the already safety-filtered label list
• prevent an unsafe preferred label from leaking into contributor-facing guidance
• cover mixed safe/unsafe and all-safe label lists

Fixes #10293

Verification

• focused prequeue preferred-label safety regression harness: passed
• relay-compatible unified patch apply/whitespace check: passed
• full LoopOver Vitest suite could not be run in the execution container because the repository dependency toolchain is unavailable here; draft CI should confirm it