Track issue promotion in organization field

[mberrys]

Summary

• Update the organization-wide Promotion stage issue field when linked work reaches dev, unstable, or stable.
• Retire the legacy in promotion queue label and stable-push issue auto-close logic; issue acceptance remains separate from branch presence.
• Serialize promotion runs, preserve other issue-field values, and fail before writes if evidence or field reads are incomplete.

Verification

• python -m scripts.github.test_issue_promotion — 11 tests passed.
• python scripts/agent/check-change.py --base origin/dev — passed.
• Live read of the issue-field values endpoint on loop2 issue L01-01 — Pin reset truth and evidence obligations #15 returned the expected empty list.

Remaining validation

The issue-field write will be exercised by the hosted workflow after this change reaches dev. GitHub's separate linked-PR auto-close setting is not changed by this PR.

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