feat(github): add GitHub Issues task channel and parity follow-ups

## Summary

Parent tracker for adding GitHub Issues as an ABCA task channel and bringing it to parity with the existing Linear/Jira issue-tracker workflows.

## Sub-issues

- [ ] #581 - Label-triggered GitHub Issues task channel
- [ ] #584 - GitHub user linking for issue-trigger attribution
- [ ] #583 - Final status comments with cost, turns, and duration
- [ ] #582 - Issue comments and supported attachments in task context
- [ ] #585 - Parent/sub-issue orchestration
- [ ] #586 - `bgagent:decompose` / `bgagent:auto` planning labels
- [ ] #587 - `@bgagent` comment-triggered PR iteration
- [ ] #588 - Extend existing orchestrations when new sub-issues are added

## Notes

- Existing upstream GitHub work is adjacent only: deployment-status screenshots, repo onboarding, credential identity, and PR-native triggers.
- This parent intentionally targets `github.com`; GitHub Enterprise Server is not tracked here.
- Native GitHub sub-issue links should mirror the checklist above.

Inizia dalle sub-issue #581–#588 per comprendere i singoli workflow di GitHub Issues e le attività successive di parità; l’issue segnala inoltre attività upstream adiacenti di GitHub che sono fuori ambito. Il lavoro è considerato completato quando le funzionalità tracciate sono implementate per github.com, la checklist è completa e i link nativi di GitHub alle sub-issue lo rispecchiano.

Scritto dal modello di indicizzazione a partire dal testo della issue.