feat(signals): comment on the source GitHub issue when an inbox report starts tracking it

Problem GitHub issues flow into the Self-driving inbox as signals, get grouped into a report, researched, and the result lands in the inbox. Nothing flows back. The people watching the GitHub issue have no idea PostHog picked it up, so they cannot find the report, and they may duplicate the investigation. We already close this loop for support tickets: products/signals/backend/support_writeback.py posts a private note on each Conversations ticket that fed a report, once the report is READY. This issue is the same behavior for GitHub issues. Linear and Jira are follow-ups (see "Out of scope"). Proposed behavior When a report reaches READY and one or more of its signals came from a GitHub issue, post one comment on each of those issues: > PostHog is tracking this issue in a self-driving inbox report. > > Report: Rules: • Opt-in per team, default off. A bot comment is visible to the customer's whole org, and on a public repo it is visible to the worl…