fix: only skip mergeability checks for merged PRs, not closed PRs

Addresses review comment: A closed-but-unmerged PR should still have
its mergeability checked accurately. Only merged PRs should skip the
checks since they're already merged.

Changes:
- SCM plugin: Only return clean status for state === "merged"
- Dashboard: Show conflicts for closed PRs (pr.state !== "merged")
- Tests: Updated to verify closed PRs still get checked

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

packages/plugins/scm-github/src/index.ts

@@ -506,11 +506,12 @@ function createGitHubSCM(): SCM {
     async getMergeability(pr: PRInfo): Promise<MergeReadiness> {
       const blockers: string[] = [];
 
-      // First, check if the PR is merged or closed
-      // GitHub returns mergeable=null for merged/closed PRs, which is not useful
+      // First, check if the PR is merged
+      // GitHub returns mergeable=null for merged PRs, which is not useful
+      // Note: We only skip checks for merged PRs. Closed PRs still need accurate status.
       const state = await this.getPRState(pr);
-      if (state === "merged" || state === "closed") {
-        // For merged/closed PRs, return a clean result without querying mergeable status
+      if (state === "merged") {
+        // For merged PRs, return a clean result without querying mergeable status
         return {
           mergeable: true,
           ciPassing: true,

packages/plugins/scm-github/test/index.test.ts

@@ -552,20 +552,18 @@ describe("scm-github plugin", () => {
       expect(ghMock).toHaveBeenCalledTimes(1);
     });
 
-    it("returns clean result for closed PRs without querying mergeable status", async () => {
+    it("still checks mergeability for closed PRs (not merged)", async () => {
       // getPRState call
       mockGh({ state: "CLOSED" });
+      // PR view (closed PRs still get checked)
+      mockGh({ mergeable: "CONFLICTING", reviewDecision: "APPROVED", mergeStateStatus: "DIRTY", isDraft: false });
+      // CI checks
+      mockGh([]);
 
       const result = await scm.getMergeability(pr);
-      expect(result).toEqual({
-        mergeable: true,
-        ciPassing: true,
-        approved: true,
-        noConflicts: true,
-        blockers: [],
-      });
-      // Should only call gh once (for getPRState), not for mergeable/CI
-      expect(ghMock).toHaveBeenCalledTimes(1);
+      expect(result.noConflicts).toBe(false);
+      expect(result.blockers).toContain("Merge conflicts");
+      // Closed PRs go through normal checks, unlike merged PRs
     });
 
     it("returns mergeable when everything is clear", async () => {

packages/web/src/components/SessionDetail.tsx

@@ -450,8 +450,8 @@ function IssuesList({ pr }: { pr: DashboardPR }) {
     });
   }
 
-  // Only show merge conflicts for open PRs (merged/closed PRs don't have mergeable status)
-  if (pr.state === "open" && !pr.mergeability.noConflicts) {
+  // Only show merge conflicts for open/closed PRs (merged PRs don't have meaningful mergeable status)
+  if (pr.state !== "merged" && !pr.mergeability.noConflicts) {
     issues.push({
       icon: "\u2717",
       color: "var(--color-accent-red)",