Add Slack notification system for GitLab CI pipelines

• slack_core: SlackClient, GitLabPipelineClient, state management
• slack_job_notif: GenericJobNotifier, TrunkCheckNotifier, NightlyNotifier
• Templates/Slack: Jinja2 templates for messages
• .gitlab-ci.yml: Pipeline with build, test, notify stages
• notify_slack.py: Main notification script