fix(db): prevent sqlite migration crash on sub_chats drop column (#100)

Prevent SQLite migration crash on startup.

`0005_marvelous_master_chief.sql` may run `ALTER TABLE sub_chats DROP COLUMN ...` on DBs where these columns never existed, causing `SQLITE_ERROR: no such column` and aborting migrations. Remove the unsafe DROP COLUMN statements (keep index creation).

Related: 15ee551 (PR #87)
- https://github.com/paul-bouzian/1code/blob/15ee55107f3eefbf012fb9754484f987b9fbf399/drizzle/0005_marvelous_master_chief.sql#L1-L4
- https://github.com/paul-bouzian/1code/blob/15ee55107f3eefbf012fb9754484f987b9fbf399/drizzle/meta/_journal.json#L40-L46

Author: a3my

drizzle/0005_marvelous_master_chief.sql

@@ -1,4 +1 @@
-CREATE INDEX `chats_worktree_path_idx` ON `chats` (`worktree_path`);--> statement-breakpoint
-ALTER TABLE `sub_chats` DROP COLUMN `additions`;--> statement-breakpoint
-ALTER TABLE `sub_chats` DROP COLUMN `deletions`;--> statement-breakpoint
-ALTER TABLE `sub_chats` DROP COLUMN `file_count`;
+CREATE INDEX `chats_worktree_path_idx` ON `chats` (`worktree_path`);