fix: require first merged PR message variable

[vitormattos]

Summary

Makes the organization first-merged-pr-comment workflow consume the repository Actions variable FIRST_MERGED_PR_MESSAGE directly.

The previous template defined an env fallback message inside the workflow. That contradicted the intended configuration model: repositories should own the complete message through an Actions variable, while the shared workflow/action only provides generic behavior and placeholders.

After this change:

• FIRST_MERGED_PR_MESSAGE is a required repository Actions variable;
• the workflow passes ${{ vars.FIRST_MERGED_PR_MESSAGE }} directly to the action;
• there is no hardcoded or generic fallback message;
• a missing/empty variable fails in the action instead of posting unexpected content;
• documentation now points to Settings → Secrets and variables → Actions → Variables;
• tests assert that the workflow has no inline fallback.

Signed-off-by: Vitor Mattos vitor@php.rio