CI: make lint job blocking — ruff backlog cleared on maintenance/gramps60

Drops the lint job's continue-on-error: true (and its TODO comment),
making ruff E9/F63/F7/F82 a blocking gate per the comment's own
instruction.

The ruff backlog the comment guarded against was cleared by 24 PRs
(gramps-project#843 + gramps-project#847-gramps-project#869) merged into maintenance/gramps60 between 2026-05-12
and 2026-05-18. Verified locally:

    pipx run ruff check --select=E9,F63,F7,F82 \
        --no-fix --exclude='*.gpr.py' .

on `gramps-project/addons-source:maintenance/gramps60` reports
"All checks passed!".

Other continue-on-error gates in this workflow (addon-structure,
compile-check, integration) each guard their own backlogs and stay
non-blocking until those clear separately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: eduralph

.github/workflows/ci.yml

@@ -16,9 +16,6 @@ jobs:
   lint:
     name: Lint
     runs-on: ubuntu-latest
-    # Non-blocking until the existing ruff E9/F63/F7/F82 errors across the
-    # addon set are cleaned up in a follow-up PR. Flip this off in that PR.
-    continue-on-error: true
     container:
       image: ghcr.io/${{ github.repository }}/gramps-ci:gramps60
     steps: