fix(status): detect merged parent branches and suggest stack sync

AGENTS.md

@@ -0,0 +1 @@
+CLAUDE.md

CLAUDE.md

@@ -119,3 +119,14 @@ Test patterns:
 ## Releases
 
 Releases are automated via GitHub Actions. When PRs are merged to main, a release is created automatically. Do not create releases manually.
+
+## Decision Log
+
+At the start of a session, read `decision-log.md` in the repo root for past architectural and design decisions.
+
+When making an architectural or design decision, add an entry to `decision-log.md` with:
+
+- **Date** (YYYY-MM-DD)
+- **Decision** — what was decided
+- **Context** — why it came up, what the problem was
+- **Resolution** — what was done and how

cmd/status.go

@@ -315,6 +315,23 @@ func detectSyncIssues(gitClient git.GitClient, stackBranches []stack.StackBranch
 			continue
 		}
 
+		// Check if parent PR is merged (or parent branch merged via git history)
+		parentPR := prCache[branch.Parent]
+		if parentPR != nil && parentPR.State == "MERGED" {
+			if verbose {
+				fmt.Printf("  ✗ Parent PR #%d has been merged\n", parentPR.Number)
+			}
+			issues = append(issues, fmt.Sprintf("  - Branch '%s': parent '%s' has been merged — run '%s' to reparent", ui.Branch(branch.Name), ui.Branch(branch.Parent), ui.Command("stack sync")))
+		} else if parentPR == nil && branch.Parent != baseBranch {
+			// No PR found for parent - check if parent was merged via git history
+			if merged, err := gitClient.IsAncestor(branch.Parent, "origin/"+baseBranch); err == nil && merged {
+				if verbose {
+					fmt.Printf("  ✗ Parent %s appears merged into %s via git history\n", branch.Parent, baseBranch)
+				}
+				issues = append(issues, fmt.Sprintf("  - Branch '%s': parent '%s' has been merged — run '%s' to reparent", ui.Branch(branch.Name), ui.Branch(branch.Parent), ui.Command("stack sync")))
+			}
+		}
+
 		// Check if PR base matches the configured parent (if PR exists)
 		if pr, exists := prCache[branch.Name]; exists {
 			if verbose {

cmd/status_test.go

@@ -148,6 +148,38 @@ func TestDetectSyncIssues(t *testing.T) {
 			},
 			expectedIssues: 1,
 		},
+		{
+			name: "parent PR is merged",
+			stackBranches: []stack.StackBranch{
+				{Name: "feature-b", Parent: "feature-a"},
+			},
+			prCache: map[string]*github.PRInfo{
+				"feature-a": {Number: 10, State: "MERGED", Base: "main"},
+				"feature-b": {Number: 11, State: "OPEN", Base: "feature-a"},
+			},
+			setupMocks: func(mockGit *testutil.MockGitClient) {
+				// PR base still matches parent, but parent is merged — should report merged parent issue
+				mockGit.On("IsCommitsBehind", "feature-b", "feature-a").Return(false, nil)
+				mockGit.On("RemoteBranchExists", "feature-b").Return(false)
+			},
+			expectedIssues: 1, // merged parent
+		},
+		{
+			name: "parent branch merged via git history (no PR for parent)",
+			stackBranches: []stack.StackBranch{
+				{Name: "feature-b", Parent: "feature-a"},
+			},
+			prCache: map[string]*github.PRInfo{
+				"feature-b": {Number: 11, State: "OPEN", Base: "feature-a"},
+			},
+			setupMocks: func(mockGit *testutil.MockGitClient) {
+				// Parent has no PR, but is ancestor of origin/main
+				mockGit.On("IsAncestor", "feature-a", "origin/main").Return(true, nil)
+				mockGit.On("IsCommitsBehind", "feature-b", "feature-a").Return(false, nil)
+				mockGit.On("RemoteBranchExists", "feature-b").Return(false)
+			},
+			expectedIssues: 1, // merged parent via git history
+		},
 	}
 
 	for _, tt := range tests {

decision-log.md

@@ -0,0 +1,11 @@
+# Decision Log
+
+Architectural and design decisions for Stackinator.
+
+## 2026-03-06 — Add merged-parent detection to `stack status`
+
+**Decision**: Add merged-parent detection to `stack status`.
+
+**Context**: `stack status` reported "perfectly synced" even when a parent branch's PR had been merged. `stack sync` already had this detection logic (checking PR state + git ancestor fallback) but `status` did not, so users had no warning until they ran sync.
+
+**Resolution**: Mirror `sync.go`'s merged-parent detection (PR state check + git ancestor fallback) in `status.go:detectSyncIssues()`. This lets `status` report when a parent branch has been merged and the child needs re-parenting.