fix(migrate-v2): don't SELECT is_main from v1 registered_groups

The v2 DB seed queried `is_main` from the v1 `registered_groups` table, but
that column was a later v1 addition — older v1 installs (e.g. 1.1.0) don't have
it, so the migration's `1b-db` step crashes with `no such column: is_main` and
v2.db is never created, cascading into the sessions and tasks steps failing.

`is_main` was selected into the V1Group interface but never read anywhere, so
this just drops it from the SELECT and the interface. The accompanying comment
already states the intent ("Query only the columns we know exist in all v1
installs") — the code now matches it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

setup/migrate-v2/db.ts

@@ -43,7 +43,6 @@ interface V1Group {
   folder: string;
   trigger_pattern: string | null;
   requires_trigger: number | null;
-  is_main: number | null;
 }
 
 async function main(): Promise<void> {
@@ -65,7 +64,7 @@ async function main(): Promise<void> {
   // v1 schema varies — channel_name was a late addition. Query only the
   // columns we know exist in all v1 installs.
   const v1Groups = v1Db
-    .prepare('SELECT jid, name, folder, trigger_pattern, requires_trigger, is_main FROM registered_groups')
+    .prepare('SELECT jid, name, folder, trigger_pattern, requires_trigger FROM registered_groups')
     .all() as V1Group[];
   v1Db.close();