fix(session-manager): refuse to restore sessions with merged/closed PRs

When a session's PR has reached a terminal state (merged or closed) and
the session is later restored, two things conspire to attach the session
to an unrelated PR:

  1. The original head branch is typically deleted on merge, so on
     workspace restore `session.branch` may drift to the project default
     branch (or be rewritten by an agent-side `git checkout main`).
  2. With branch == default branch, downstream `detectPR` matches
     whatever PR happens to share that head ref name (including fork PRs).

Fix 1 (scm-github) and Fix 2 (lifecycle) defend in depth, but the cleanest
guarantee is to refuse the restore up-front: a session whose PR has
already merged/closed has no remaining work to do. If the user wants
follow-up work, they should spawn a new session.

The previous behavior — silently clearing terminal `lifecycle.pr.*`
fields on restore and resetting `session.state` to "working" — is removed
because it's now unreachable: the new check at step 3a throws before
we get there.

Invariants preserved:
- Workers with `pr.state` of "open"/"none" still restore normally.
- `isRestorable` semantics unchanged for terminated/done/runtime-lost.
- Errors surface via `SessionNotRestorableError`, the same channel used
  by other restore-blockers (workspace missing, OpenCode mapping, etc).

Refs #1724.

packages/core/src/__tests__/session-manager/restore.test.ts

@@ -197,12 +197,13 @@ describe("restore", () => {
     expect(mockRuntime.create).toHaveBeenCalled();
   });
 
-  it("allows restoring merged sessions", async () => {
+  it("refuses to restore sessions whose PR has merged (would silently re-spawn against the default branch)", async () => {
     const ws = "/tmp/mock-ws/app-1";
     writeMetadata(sessionsDir, "app-1", {
       worktree: ws,
       branch: "main",
       status: "merged",
+      pr: "https://github.com/acme/repo/pull/1723",
       project: "my-app",
       runtimeHandle: makeHandle("rt-old"),
     });
@@ -222,8 +223,7 @@ describe("restore", () => {
     };
 
     const sm = createSessionManager({ config, registry });
-    const restored = await sm.restore("app-1");
-    expect(restored.id).toBe("app-1");
+    await expect(sm.restore("app-1")).rejects.toThrow(/is merged/);
   });
 
   it("throws SessionNotRestorableError for working sessions", async () => {

packages/core/src/session-manager.ts

@@ -2801,6 +2801,23 @@ export function createSessionManager(deps: SessionManagerDeps): OpenCodeSessionM
       throw new SessionNotRestorableError(sessionId, reason);
     }
 
+    // 3a. Refuse to silently revive a session whose PR has reached a terminal
+    //     state. The original branch has typically been deleted on merge, so
+    //     `session.branch` may have drifted to the project default branch
+    //     between the kill and the restore. Re-spawning would associate the
+    //     session with whatever PR `detectPR` finds on that branch — including
+    //     unrelated fork PRs (see #1724). If the user wants a fresh session
+    //     in this workspace, they should spawn a new one.
+    const prState = session.lifecycle.pr.state;
+    if (prState === "merged" || prState === "closed") {
+      const prRef = session.lifecycle.pr.url ?? session.pr?.url ?? raw["pr"] ?? "(unknown)";
+      throw new SessionNotRestorableError(
+        sessionId,
+        `PR ${prRef} is ${prState}; the session's work is complete. ` +
+          `Spawn a new session for follow-up work instead of restoring.`,
+      );
+    }
+
     // 4. Validate required plugins (plugins already resolved above for enrichment)
     if (!plugins.runtime) {
       throw new Error(`Runtime plugin '${project.runtime ?? config.defaults.runtime}' not found`);
@@ -2967,16 +2984,8 @@ export function createSessionManager(deps: SessionManagerDeps): OpenCodeSessionM
     restoredLifecycle.runtime.handle = handle;
     restoredLifecycle.runtime.lastObservedAt = now;
 
-    // Reset terminal PR state so the lifecycle manager doesn't immediately
-    // re-terminate the session. The old PR is done — if the agent creates
-    // a new one, PR auto-detect will pick it up.
-    if (restoredLifecycle.pr.state === "merged" || restoredLifecycle.pr.state === "closed") {
-      restoredLifecycle.pr.state = "none";
-      restoredLifecycle.pr.reason = "cleared_on_restore";
-      restoredLifecycle.pr.number = null;
-      restoredLifecycle.pr.url = null;
-      restoredLifecycle.pr.lastObservedAt = null;
-    }
+    // Terminal PR state (merged/closed) is rejected up-front in step 3a, so
+    // by here the lifecycle.pr is non-terminal and is preserved as-is.
 
     updateMetadata(sessionsDir, sessionId, {
       ...buildLifecycleMetadataPatch(restoredLifecycle),