Carry the worktree head on PR-refresh candidates so fallback preservation is head-gated

Live validation caught a self-sustaining loop the unit seams missed: the
Checks panel passes its displayed PR number as the refresh fallback, and
the candidate path carried no worktree head, so main preserved a merged
branch PR head-blind — the merged view survived new local commits instead
of yielding to 'No pull request found'.

Thread currentHeadOid through GitHubPRRefreshCandidate, both renderer
candidate builders, the gh:prForBranch IPC, and the runtime RPC method —
the same shape the hosted-review path already used. Main's explicit-head
gate (equality or confirmed commit membership) now applies to every
renderer-driven lookup; true no-context callers keep the designed
deleted-head preservation.

Co-authored-by: Orca <help@stably.ai>

src/main/github/pr-refresh-coordinator.test.ts

@@ -94,6 +94,28 @@ describe('pr-refresh-coordinator', () => {
     vi.useRealTimers()
   })
 
+  it('forwards the candidate worktree head into the branch lookup options', async () => {
+    const { reportVisiblePRRefreshCandidates } = await import('./pr-refresh-coordinator')
+    getPRForBranchOutcomeMock.mockResolvedValueOnce({
+      kind: 'no-pr',
+      fetchedAt: Date.now()
+    })
+
+    reportVisiblePRRefreshCandidates([makeCandidate({ currentHeadOid: 'worktree-head-oid' })], 1, 1)
+    await vi.runOnlyPendingTimersAsync()
+
+    // Why: without the head, a panel-supplied fallback number preserves a
+    // merged PR head-blind after the branch moves on to new work.
+    expect(getPRForBranchOutcomeMock).toHaveBeenCalledWith(
+      '/repo',
+      'feature/test',
+      null,
+      null,
+      null,
+      expect.objectContaining({ currentHeadOid: 'worktree-head-oid' })
+    )
+  })
+
   it('does not show visible background refreshes as queued', async () => {
     const { reportVisiblePRRefreshCandidates } = await import('./pr-refresh-coordinator')
     getPRForBranchOutcomeMock.mockResolvedValueOnce({

src/main/github/pr-refresh-coordinator.ts

@@ -34,7 +34,7 @@ type PRRefreshOutcomeObserver = (
 
 type PRBranchLookupCandidate = Pick<
   GitHubPRRefreshCandidate,
-  'localGitOptions' | 'linkedPRNumber' | 'fallbackPRNumber' | 'fallbackPRSource'
+  'localGitOptions' | 'linkedPRNumber' | 'fallbackPRNumber' | 'fallbackPRSource' | 'currentHeadOid'
 >
 
 function shouldAcceptMergedFallbackPR(candidate: PRBranchLookupCandidate): boolean {
@@ -55,6 +55,9 @@ function hostedReviewOptionArgs(
   if (shouldAcceptMergedFallbackPR(candidate)) {
     options.acceptMergedFallbackPR = true
   }
+  if (typeof candidate.currentHeadOid === 'string' && candidate.currentHeadOid.trim().length > 0) {
+    options.currentHeadOid = candidate.currentHeadOid
+  }
   return Object.keys(options).length > 0 ? [options] : []
 }