perf(forge): compare from the recorded base while confirming the head

Status snapshots carry the base commit the host recorded with the head
(baseSha). ForgeService.diff passes it to the provider when the latest
status names the requested head, and forge-github starts the compare read
from it while its GraphQL read confirms the head; a stale head or another
base cancels and joins the early read.

packages/experimental/forge-github/src/graphql.ts

@@ -32,7 +32,7 @@ const THREAD_FIELDS = 'pageInfo { hasNextPage endCursor } nodes { isResolved }'
 function pullRequestFields(threads: number): string {
   return `id number url title state isDraft updatedAt closedAt
   author { login }
-  headRefName headRefOid baseRefName additions deletions mergeable mergeStateStatus reviewDecision
+  headRefName headRefOid baseRefName baseRefOid additions deletions mergeable mergeStateStatus reviewDecision
   reviewThreads(first: ${threads}) { ${THREAD_FIELDS} }
   commits(last: 1) { nodes { commit { statusCheckRollup { id state contexts(first: ${PAGE_SIZE}) {
     ${CHECK_FIELDS}
@@ -132,6 +132,7 @@ const pullRequestSchema = z.object({
   headRefName: z.string(),
   headRefOid: z.string(),
   baseRefName: z.string(),
+  baseRefOid: z.string().refine(isCommit),
   additions: z.number().int().nonnegative(),
   deletions: z.number().int().nonnegative(),
   mergeable: z.string(),
@@ -466,6 +467,7 @@ function toSnapshot(
     headBranch: node.headRefName,
     headSha: node.headRefOid,
     baseBranch: node.baseRefName,
+    baseSha: node.baseRefOid,
     additions: node.additions,
     deletions: node.deletions,
     mergeability: mergeability(node.mergeable),