fix: Handle spawn and branch failures gracefully in dispatch cycle

Wrap agentManager.spawn() in try/catch — on failure, block the task
instead of crashing the entire dispatch cycle. Move phase status update
to after branch creation succeeds — on branch failure, block the phase
and skip task queuing. Fix statement-breakpoint markers in migration
0020 to use separate lines.

apps/server/drizzle/0020_add_change_sets.sql

@@ -9,7 +9,8 @@ CREATE TABLE `change_sets` (
   `status` text NOT NULL DEFAULT 'applied',
   `reverted_at` integer,
   `created_at` integer NOT NULL
-);--> statement-breakpoint
+);
+--> statement-breakpoint
 CREATE TABLE `change_set_entries` (
   `id` text PRIMARY KEY NOT NULL,
   `change_set_id` text NOT NULL REFERENCES `change_sets`(`id`) ON DELETE CASCADE,
@@ -20,6 +21,8 @@ CREATE TABLE `change_set_entries` (
   `new_state` text,
   `sort_order` integer NOT NULL DEFAULT 0,
   `created_at` integer NOT NULL
-);--> statement-breakpoint
-CREATE INDEX `change_sets_initiative_id_idx` ON `change_sets`(`initiative_id`);--> statement-breakpoint
+);
+--> statement-breakpoint
+CREATE INDEX `change_sets_initiative_id_idx` ON `change_sets`(`initiative_id`);
+--> statement-breakpoint
 CREATE INDEX `change_set_entries_change_set_id_idx` ON `change_set_entries`(`change_set_id`);