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.

Start with sub-issues #581–#588 to understand the individual GitHub Issues workflows and parity follow-ups; the issue also notes adjacent upstream GitHub work that is out of scope. Done means the tracked capabilities are implemented for github.com, the checklist is complete, and native GitHub sub-issue links mirror it.

Written by the indexing model from the issue text.