feat(workflow): route intake and hand triage to duty engineer

[purp]

Summary

Label new issues at intake and give human triage engineers a private Slack handoff path, while leaving automatic triage review for the linked follow-on issue.

Related Issue

Closes #3811
Part of #3807
Stacked on #3810

Changes

• Route maintainer-authored issues to state:accepted, and external issues to state:new.
• Add a human-invoked triage handoff that posts summaries in the OpenShell triage channel and mentions the duty engineer group.
• Update the triage skill for collaborative human disposition and safe security handling.
• Keep Slack bot token and channel/group IDs configurable; bot installation and token provision remain rollout steps.

Testing

• mise run pre-commit passes
• Unit tests added/updated for intake and Slack handoff
• E2E tests added/updated (not applicable without Slack credentials)
• mise run ci passes with an isolated Go lint cache

Checklist

• Follows Conventional Commits
• Commits are signed and signed off (DCO)
• Architecture docs updated (not applicable; contributor workflow docs updated)

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[purp]

/ok to test ff008a5

[purp]

/ok to test 3d2de58