Ensure reactive commands don't match arguments on next line

What does this MR do and why?

Previously, the reactive command matcher would use [[:space:]] which includes carriage returns:

/[[:space:]]/ - Whitespace character ([:blank:], newline, carriage return, etc.)

By using [[:blank:]] instead, we make sure we only match on the same line:

/[[:blank:]]/ - Space or tab

Expected impact & dry-runs

Closes #1040 (closed).

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review