feat(upstream): import open PRs as provisional snapshots

[incognitojam]

Open upstream PRs currently require importing every constituent commit with Upstream-Commit provenance, because an Upstream-PR trailer would make the queue skip the eventual upstream merge. Open-PR intake now uses one attributed snapshot commit with Upstream-PR, followed by a separate tracking commit recording the reviewed base and head SHAs and the reconciliation plan.

Provisional markers keep the final integration pending until explicit reconciliation and cleanup. The intake audit requires the marker and compares the implementation with the frozen aggregate diff. Closed imports are flagged for review. Queue and candidate reports read committed tracking metadata from the selected fork ref, and lag history records completion at cleanup rather than retroactively at the snapshot import. The runbook and feature ledger describe this workflow, including reconciliation when a PR merges, closes, or reopens.

Validation: 54 focused upstream tests passed, including a disposable Git repository running the actual audit, queue, and tracked-report commands. A two-commit source matched the single snapshot's aggregate diff; provisional imports stayed selectable and blocked baseline advancement; draft tracking edits did not change committed-ref reports; an identical final change completed only at cleanup; and a closed-import decision did not account for a later reopened merge. Scripts typecheck, scoped lint, and the fork feature ledger check passed.

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Written by an agent (Codex, GPT-6).

[github-actions]

Upstream divergence

Both sides are compared with upstream at the in-order tip 567783ecdc. Excludes .repos/ and pnpm-lock.yaml.

Measure	Base	This PR	Change
Upstream files the fork changes	1,188	1,188	0
Lines changed in them	63,615	63,615	0
Upstream files the fork removes	37	37	0
Fork-only files	344	345	+1
Fork-only lines	48,099	48,752	+653

No upstream file changes how much it differs from upstream.