feat(review): add managed labeling runtime foundation #5406

User Story

As an OpenShell maintainer preparing automatic labeling of accepted issues and PRs, I need a reviewable runtime adapter and invocation selector that reuse the standalone /label-issue workflow, so we can validate the managed execution boundary before enabling automatic application.

Description

Deliver the runtime foundation for Part 3 of #5276: a Claude Managed Agents classification adapter, deterministic selection of acceptance events and authorized human invocations, offline regression coverage, and concise integration documentation.

The adapter consumes prepared evidence and trusted classification instructions and returns a proposal with an execution reference. The shared helper from #5355 retains eligibility, evidence preparation/expansion, semantic validation, label writes, checkpoints, and local reporting. Include an explicit request-preview/live smoke-test harness; do not activate automatic labeling as part of this foundation.

Context

• feat(review): automatically label new PRs for accepted work #5260 supplies the accepted-work definition/library and applies acceptance to qualifying newly opened PRs, including drafts. It does not make triage accept work or continuously refresh acceptance.
• feat(skills): extract issue and PR labeling #5355 extracts the standalone labeling skill and per-item migration/application helpers.
• chore: prepare area and topic labels for migration #5151 prepares canonical labels, retirement guidance, and routing support before manual migration.
• The eventual automatic trigger remains explicit acceptance, not item creation or reopening. The GITHUB_TOKEN acceptance writer needs an explicit invocation hook because its label event does not launch another workflow.
• CMA configuration, shared-helper wiring, coordinated per-item execution, live observed runs, and automatic application remain follow-up work tracked in chore: create an invocable agent for workflow and topic labeling #5276. Do not close that overall tracker with the foundation PR.

Definition of Done

• Provide a version-pinned, budget/deadline-bounded classifier adapter returning an unvalidated JSON proposal and session reference.
• Keep tools, GitHub writes, eligibility, and checkpoint logic outside the remote classifier; preserve failed-session recovery and uncertain-write handling.
• Select issue/PR acceptance events and authorized human PR-comment/manual invocations, including force, without independently deciding acceptance.
• Cover completion, failures, pagination, human authorization, bot-applied acceptance, drafts, and request preview with offline tests registered in CI.
• Document the shared-helper handoff and remaining activation prerequisites; install no automatic labeling workflow in this foundation.