fix(workflows): replace invalid trim() in LGTM matcher [DOPS-600]

[kevnm67]

trim() is not a valid GHA expression — silently broke LGTM-by-comment merge. See LocumTenens-com/ltx#117 for source fix.

Test plan

• Comment LGTM on a PR → workflow fires

[sentry]

Bug: The workflow trigger condition incorrectly uses a substring match for "LGTM", which can cause it to run on non-approval comments.
_{Severity: MEDIUM}

Suggested Fix

Revert to the previous logic using contains(fromJSON('["LGTM", "lgtm", "Lgtm"]'), trim(github.event.comment.body)) to perform an exact match against a list of approval strings. This prevents unintended triggers from comments that only contain the approval text as a substring.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: .github/workflows/merge-on-green.yml#L24

Potential issue: The workflow trigger condition was changed from an exact match against
an array of approval strings to a substring search. This is overly permissive and will
cause the merge workflow to trigger on comments that are not approvals but contain the
substring "LGTM", such as "This is NOT LGTM". Additionally, the mixed-case `Lgtm`
variant was removed from the condition, so that specific approval comment will no longer
trigger the workflow.

_{Did we get this right? 👍 / 👎 to inform future reviews.}