feat(ms22-p2): add UserAgent CRUD endpoints

- UserAgentService with findAll, findOne, create, update, remove
- UserAgentController at /api/agents (authenticated, user-scoped)
- createFromTemplate endpoint to instantiate from AgentTemplate
- Update TASKS.md and MISSION-MANIFEST.md for P2-004

Task: MS22-P2-004

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/user-agent/dto/create-user-agent.dto.ts

@@ -0,0 +1,43 @@
+import { IsString, IsBoolean, IsOptional, IsArray, MinLength } from "class-validator";
+
+export class CreateUserAgentDto {
+  @IsString()
+  @MinLength(1)
+  templateId?: string;
+
+  @IsString()
+  @MinLength(1)
+  name!: string;
+
+  @IsString()
+  @MinLength(1)
+  displayName!: string;
+
+  @IsString()
+  @MinLength(1)
+  role!: string;
+
+  @IsString()
+  @MinLength(1)
+  personality!: string;
+
+  @IsString()
+  @IsOptional()
+  primaryModel?: string;
+
+  @IsArray()
+  @IsOptional()
+  fallbackModels?: string[];
+
+  @IsArray()
+  @IsOptional()
+  toolPermissions?: string[];
+
+  @IsString()
+  @IsOptional()
+  discordChannel?: string;
+
+  @IsBoolean()
+  @IsOptional()
+  isActive?: boolean;
+}