fix: retain snapshots for removed workspaces

backend/src/workspaces/registry.ts

@@ -141,6 +141,31 @@ export class WorkspaceRegistry {
     return (await this.activeState()).revisions;
   }
 
+  /**
+   * List every intact retained snapshot, current snapshots first. Session discovery and
+   * retention use this rather than only the active revision so removing a workspace from
+   * Git cannot strand a resumable session that still pins one of its older descriptors.
+   */
+  async listRetainedSnapshots(): Promise<WorkspaceRevision[]> {
+    await this.repository.ensureLayout();
+    return await this.lock.run(async () => {
+      try {
+        const active = await this.activeState();
+        const revisions = [...active.revisions];
+        const entries = await readdir(this.repository.snapshotsPath, { withFileTypes: true });
+        for (const entry of entries) {
+          if (!entry.isDirectory() || entry.isSymbolicLink() || !/^[0-9a-f]{40}$/.test(entry.name)) continue;
+          if (entry.name === active.head) continue;
+          const state = await this.snapshotState(entry.name);
+          revisions.push(...state.revisions.filter((revision) => revision.state === "operational"));
+        }
+        return revisions;
+      } catch (error) {
+        throw workspaceError(error);
+      }
+    });
+  }
+
   async read(id: string): Promise<{ workspace: WorkspaceDescriptor; revision: WorkspaceRevision }> {
     const state = await this.activeState();
     const revision = state.revisions.find((candidate) => candidate.id === id);
@@ -550,6 +575,19 @@ export class WorkspaceRegistry {
     return JSON.parse(await readFile(path, "utf8"));
   }
 
+  private async snapshotState(head: string): Promise<ActiveState> {
+    const manifest = await this.readSnapshotManifest(safeCommit(head));
+    if (this.isLegacySnapshotManifest(manifest)) {
+      const state = await this.deriveStateFromLegacyRevisions(manifest);
+      await this.migrateLegacySnapshotManifest(state, manifest);
+      return state;
+    }
+    const state = manifest as ActiveState;
+    this.assertActiveState(state);
+    await this.assertSnapshotIntegrity(state);
+    return state;
+  }
+
   private async assertSnapshotIntegrity(state: ActiveState): Promise<void> {
     const directory = join(this.repository.snapshotsPath, state.head);
     try {