test: mirror the prober's git invocation in the CI diagnostic

The first diagnostic round proved the prober's inline snapshot returns
no branch on hosted runners while the fixture's own git calls succeed;
capture the mirrored invocation's exit status and stderr to name the
cause.

Author: arzafran

programaTests/TabManagerUnitTests.swift

@@ -618,11 +618,28 @@ final class TabManagerPullRequestProbeTests: XCTestCase {
             ) ?? "UNRESOLVED"
             let inlineSnapshot = GitMetadataProber.initialWorkspaceGitMetadataSnapshot(for: repoURL.path)
             let probePanels = manager.activeWorkspaceGitProbePanelIdsForTesting(workspaceId: backgroundWorkspace.id)
+            // Mirror the prober's exact invocation and surface status + stderr.
+            let mirror = Process()
+            mirror.executableURL = URL(fileURLWithPath: gitPath)
+            mirror.arguments = ["branch", "--show-current"]
+            mirror.currentDirectoryURL = repoURL
+            let mirrorOut = Pipe(); let mirrorErr = Pipe()
+            mirror.standardOutput = mirrorOut; mirror.standardError = mirrorErr
+            var mirrorSummary = "spawn-failed"
+            do {
+                try mirror.run(); mirror.waitUntilExit()
+                let out = String(data: mirrorOut.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8) ?? ""
+                let err = String(data: mirrorErr.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8) ?? ""
+                mirrorSummary = "status=\(mirror.terminationStatus) stdout=\(out.trimmingCharacters(in: .whitespacesAndNewlines)) stderr=\(err.trimmingCharacters(in: .whitespacesAndNewlines).prefix(300))"
+            } catch {
+                mirrorSummary = "spawn error: \(error)"
+            }
             XCTFail(
                 "branch never arrived. gitPath=\(gitPath) " +
                 "inlineSnapshot(branch=\(inlineSnapshot.branch ?? "nil"), dirty=\(inlineSnapshot.isDirty)) " +
                 "modelBranch=\(backgroundWorkspace.panelGitBranches[backgroundPanelId]?.branch ?? "nil") " +
                 "activeProbes=\(probePanels.count) " +
+                "mirror[\(mirrorSummary)] " +
                 "bgWorkspaceDir=\(backgroundWorkspace.currentDirectory ?? "nil") repo=\(repoURL.path)"
             )
         }