brain/gateway: prohibit mission_tasks.status as a write source (M4-3a phase 1)

SHARED-CONTRACT §5.1 phase 1 N-1 patch, ahead of the M4-3a expand DDL:
- mission-tasks repo (sole write path) strips status on create/update;
  the column keeps its DB default and stays declared and readable.
- missions controller stops forwarding dto.status on create.
- DTO status fields stay declared (forbidNonWhitelisted would 400 frozen
  legacy consumers) but are documented deprecated/ignored.
- §5.5 write-prohibition spec: 4 tests on what reaches the Drizzle chain.

apps/gateway/src/missions/missions.controller.ts

@@ -108,11 +108,13 @@ export class MissionsController {
   ) {
     const mission = await this.brain.missions.findByIdAndUser(missionId, user.id);
     if (!mission) throw new NotFoundException('Mission not found');
+    // dto.status is deliberately not forwarded: mission_tasks.status is
+    // write-prohibited through the N-1 window (SHARED-CONTRACT §5.1 phase 1);
+    // the repo strips it as well.
     return this.brain.missionTasks.create({
       missionId,
       taskId: dto.taskId,
       userId: user.id,
-      status: dto.status,
       description: dto.description,
       notes: dto.notes,
       pr: dto.pr,

apps/gateway/src/missions/missions.dto.ts

@@ -77,6 +77,12 @@ export class CreateMissionTaskDto {
   @IsUUID()
   taskId?: string;
 
+  /**
+   * @deprecated Accepted for N-1 wire compatibility but ignored: mission_tasks.status
+   * is write-prohibited through the migration window (SHARED-CONTRACT §5.1 phase 1).
+   * The field stays declared because the global ValidationPipe runs with
+   * forbidNonWhitelisted, and removing it would 400 frozen legacy consumers.
+   */
   @IsOptional()
   @IsIn(taskStatuses)
   status?: 'not-started' | 'in-progress' | 'blocked' | 'done' | 'cancelled';
@@ -102,6 +108,12 @@ export class UpdateMissionTaskDto {
   @IsUUID()
   taskId?: string;
 
+  /**
+   * @deprecated Accepted for N-1 wire compatibility but ignored: mission_tasks.status
+   * is write-prohibited through the migration window (SHARED-CONTRACT §5.1 phase 1).
+   * The field stays declared because the global ValidationPipe runs with
+   * forbidNonWhitelisted, and removing it would 400 frozen legacy consumers.
+   */
   @IsOptional()
   @IsIn(taskStatuses)
   status?: 'not-started' | 'in-progress' | 'blocked' | 'done' | 'cancelled';