tools: apply feedback to and simplify contributor guidance workflow

[panva]

Applying feedback from #65745 (review) and, assuming contributor-guidance.yml runs confirm it's not necessary, removing and resolving its TODO.

This is also a place to apply further feedback collected from observing the action actually run.

• MEMBER skipped entirely
  • 33881584998
• MEMBER w/ private organization membership resolved to MEMBER and didn't scan or post
  • 101066515260
• CONTRIBUTOR skips posting caution
  • 33881601428
• CONTRIBUTOR posts a caution and labels
• FIRST_TIMER or FIRST_TIME_CONTRIBUTOR posts a welcome
  • 33883855448, comment
• FIRST_TIMER or FIRST_TIME_CONTRIBUTOR posts a welcome combined with a caution and labels
  • 33978161415, comment
• multiple concurrent FIRST_TIME_CONTRIBUTOR for the same user 33888126449, 33888131494
• FIRST_TIME_CONTRIBUTOR welcome only evolves into a caution on the second PR, suggesting that caching may not be desired
  • 33976864974, 33978161415, comment

Investigate:

• 33886219390 Q: why did this not get skipped?
  • A: private organization membership

Suggestions:

• Ensure label is present on all of the author's PRs? Upon classification query for other open PRs by that same author that lack the label and add it.

[nodejs-github-bot]

Review requested:

• @nodejs/actions

[aduh95]

A nice-to-have would be to also check which base branch the PR is targeting in case it's not main, but that can be a follow-up – or not, after all it's arguably a rare enough occurrence