Delegate GitHub issue triage through automation subject turns

[neubig]

Desired Behavior

The GitHub issue triage automation should remain an ordinary scanner process and delegate only the selected issue to an agent. Conversation creation, attachment, and local/Docker runtime lifecycle belong to the Automation service.

Acceptance Criteria

• The existing github-issue-triage extension scans GitHub without creating or attaching to a conversation.
• Each changed issue is submitted as an idempotent subject turn using a stable repository-and-issue identity.
• The triage agent publishes readable acceptance criteria and priority directly on GitHub using the selected profile's credentials.
• Extension code contains no Agent Server URL, session key, conversation attachment, runtime polling, or workspace-kind branching.
• Focused tests cover selection, deduplication, and subject-turn submission.
• Live Agent Canvas evidence shows the scanner outside the agent runtime and the selected triage agent running in Docker.

[github-actions]

✅ ready-for-dev label applied

This issue meets the readiness criteria and is ready for contribution.
A contributor can now open a PR that links this issue (Fixes #<number>).

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