Why the Nightly Triage Job Opens Draft PRs, Not Merged Ones

Reviewing the worked example's nightly issue-triage job, a colleague proposes a change: "Since the job already restricts itself to changes under ~20 lines with a clearly testable fix, and CI runs the test suite before opening the PR, let's have it auto-merge PRs where CI passes — that saves someone from reviewing trivial fixes every morning."

1. Explain specifically what test-suite-passing does and does not verify about a change, in terms relevant to this proposal.
2. Give a concrete example of a change that is "under 20 lines," has "a clearly testable fix," and passes the existing test suite, but would still be wrong to auto-merge.
3. What's a middle-ground guardrail that captures most of the time savings your colleague wants without removing human review entirely?