feat(signals): build maintainer burden forecast snapshots

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Problem

Maintainers care about review load, stale queues, duplicate clusters, and reviewable PRs. Individual PR advisories are not enough to understand repo-level burden.

Desired behavior

Generate repo-level burden forecast snapshots and expose them through repo intelligence and private MCP/API surfaces.

Implementation notes

• Use open PR age, stale PRs, duplicate clusters, reviewable PRs, failing checks, broad diffs, and unlinked PRs.
• Generate snapshots asynchronously; do not compute large repo forecasts inline.
• Keep ragflow/sure scale in mind.
• Store enough freshness metadata for clients to explain when forecasts are stale.

Public/private boundaries

Forecasts are private API/MCP intelligence. Public GitHub comments should stay minimal and sanitized.

Acceptance criteria

• Large queue fixtures complete within Worker-safe bounds.
• Forecasts identify top burden sources and cleanup actions.
• Repo intelligence includes freshness and data-quality warnings.
• No broad request-time scans are needed.

Tests

• Small repo queue fixture.
• Large ragflow/sure-style queue fixture.
• Duplicate cluster fixture.
• Stale PR trend fixture.
• Snapshot freshness fixture.
• Performance regression for bounded work.