fix(ci): enable conventional PR titles by default

[trakhimenok]

Conventional PR-title validation currently requires each caller to opt in, so new workflow consumers can still miss releases because a squash title is unconstrained. Make the shared gate default to enabled while retaining require_conventional_pr_title: false as an explicit opt-out.

The documentation now shows the default and opt-out forms, and the workflow contract test locks the default to true.

Validation: go test ./..., go vet ./..., and git diff --check passed.

[trakhimenok]

AI review: approved.

The diff changes only the reusable input default, its documentation, and the matching regression assertion. The conditional still honors an explicit false, so opt-out behavior remains intact. The PR's conventional title exercised the default-on path in the required Lint job, all seven exact-head checks passed, and go test ./..., go vet ./..., and git diff --check passed locally. I found no correctness or security issue.