fix(schema): guard migration 28 indexes behind tableExists

Migration 28 (add_snapshot_hot_path_indexes) referenced task_comments,
documents, meetings, sprints, agent_messages, activity — all tables
created by ensureOptionalColumns() which runs AFTER runMigrations() on
a fresh DB. Converted from sql-string migration to fn-based so each
index is gated by tableExists(); safe on both fresh and migrated DBs.

Also updated tests/unit/selective-ipc.test.ts exhaustive switch to
include the 7 new DesktopEvent variants added in ef90a58.

tests/unit/selective-ipc.test.ts

@@ -110,6 +110,20 @@ describe("selective IPC event types", () => {
           return "thread-wait-state-updated";
         case "recovery-needed":
           return "recovery-needed";
+        case "oauth:status":
+          return "oauth:status";
+        case "oauth:auth-url":
+          return "oauth:auth-url";
+        case "oauth:progress":
+          return "oauth:progress";
+        case "oauth:prompt-request":
+          return "oauth:prompt-request";
+        case "oauth:manual-code-request":
+          return "oauth:manual-code-request";
+        case "credentials-changed":
+          return "credentials-changed";
+        case "agent-event":
+          return "agent-event";
         default: {
           const _exhaustive: never = event;
           return _exhaustive;