feat(agentic): add issue→fix→review→merge agent workflow

Adds the four-agent GitLab workflow (reporter/resolver/reviewer/integrator) coordinated via scoped labels, plus gl-flow.sh dispatcher, headless cron runner, supervised agent-view launcher, helper subagents, per-role least-privilege settings, Makefile targets, and the runbook. See agent-instructions/agentic-workflow.md.