test: emit probe-chain diagnostics when the branch never arrives on CI

Three hardening strategies failed against these tests on hosted
runners; on the next failure the message reports the prober's resolved
git path, its synchronous inline snapshot, and the live probe state, so
the failing link is identified from the CI log instead of guessed.

Author: arzafran

programaTests/TabManagerUnitTests.swift

@@ -603,14 +603,29 @@ final class TabManagerPullRequestProbeTests: XCTestCase {
         }
 
         XCTAssertNotEqual(manager.selectedTabId, backgroundWorkspace.id)
-        XCTAssertTrue(
-            // Real git subprocess + GitMetadataProber round trip, not a fixed dispatch
-            // delay — needs the same headroom as testRemoteSplitSkipsInitialGitMetadataProbe
-            // below under a full serial suite run's CPU contention.
-            waitForCondition(timeout: 12.0) {
-                backgroundWorkspace.panelGitBranches[backgroundPanelId]?.branch == "main"
-            }
-        )
+        // Real git subprocess + GitMetadataProber round trip, not a fixed dispatch
+        // delay — needs the same headroom as testRemoteSplitSkipsInitialGitMetadataProbe
+        // below under a full serial suite run's CPU contention.
+        let branchArrived = waitForCondition(timeout: 12.0) {
+            backgroundWorkspace.panelGitBranches[backgroundPanelId]?.branch == "main"
+        }
+        if !branchArrived {
+            // CI diagnostic: distinguish subprocess/env failure from async-apply failure.
+            let gitPath = GitMetadataProber.resolvedCommandPathForTesting(
+                executable: "git",
+                environment: ProcessInfo.processInfo.environment,
+                fallbackDirectories: ["/usr/bin", "/opt/homebrew/bin", "/usr/local/bin", "/opt/local/bin"]
+            ) ?? "UNRESOLVED"
+            let inlineSnapshot = GitMetadataProber.initialWorkspaceGitMetadataSnapshot(for: repoURL.path)
+            let probePanels = manager.activeWorkspaceGitProbePanelIdsForTesting(workspaceId: backgroundWorkspace.id)
+            XCTFail(
+                "branch never arrived. gitPath=\(gitPath) " +
+                "inlineSnapshot(branch=\(inlineSnapshot.branch ?? "nil"), dirty=\(inlineSnapshot.isDirty)) " +
+                "modelBranch=\(backgroundWorkspace.panelGitBranches[backgroundPanelId]?.branch ?? "nil") " +
+                "activeProbes=\(probePanels.count) " +
+                "bgWorkspaceDir=\(backgroundWorkspace.currentDirectory ?? "nil") repo=\(repoURL.path)"
+            )
+        }
         XCTAssertEqual(backgroundWorkspace.sidebarGitBranchesInDisplayOrder().map(\.branch), ["main"])
     }