feat(#89): implement COMMAND message type for federation

Implements federated command messages following TDD principles and
mirroring the QueryService pattern for consistency.

## Implementation

### Schema Changes
- Added commandType and payload fields to FederationMessage model
- Supports COMMAND message type (already defined in enum)
- Applied schema changes with prisma db push

### Type Definitions
- CommandMessage: Request structure with commandType and payload
- CommandResponse: Response structure with correlation
- CommandMessageDetails: Full message details for API responses

### CommandService
- sendCommand(): Send command to remote instance with signature
- handleIncomingCommand(): Process incoming commands with verification
- processCommandResponse(): Handle command responses
- getCommandMessages(): List commands for workspace
- getCommandMessage(): Get single command details
- Full signature verification and timestamp validation
- Error handling and status tracking

### CommandController
- POST /api/v1/federation/command - Send command (authenticated)
- POST /api/v1/federation/incoming/command - Handle incoming (public)
- GET /api/v1/federation/commands - List commands (authenticated)
- GET /api/v1/federation/commands/:id - Get command (authenticated)

## Testing
- CommandService: 15 tests, 90.21% coverage
- CommandController: 8 tests, 100% coverage
- All 23 tests passing
- Exceeds 85% coverage requirement
- Total 47 tests passing (includes command tests)

## Security
- RSA signature verification for all incoming commands
- Timestamp validation to prevent replay attacks
- Connection status validation
- Authorization checks on command types

## Quality Checks
- TypeScript compilation: PASSED
- All tests: 47 PASSED
- Code coverage: >85% (90.21% for CommandService, 100% for CommandController)
- Linting: PASSED

Fixes #89

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/federation/dto/command.dto.ts

@@ -0,0 +1,54 @@
+/**
+ * Command DTOs
+ *
+ * Data Transfer Objects for command message operations.
+ */
+
+import { IsString, IsObject, IsNotEmpty, IsNumber } from "class-validator";
+import type { CommandMessage } from "../types/message.types";
+
+/**
+ * DTO for sending a command to a remote instance
+ */
+export class SendCommandDto {
+  @IsString()
+  @IsNotEmpty()
+  connectionId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  commandType!: string;
+
+  @IsObject()
+  @IsNotEmpty()
+  payload!: Record<string, unknown>;
+}
+
+/**
+ * DTO for incoming command request from remote instance
+ */
+export class IncomingCommandDto implements CommandMessage {
+  @IsString()
+  @IsNotEmpty()
+  messageId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  instanceId!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  commandType!: string;
+
+  @IsObject()
+  @IsNotEmpty()
+  payload!: Record<string, unknown>;
+
+  @IsNumber()
+  @IsNotEmpty()
+  timestamp!: number;
+
+  @IsString()
+  @IsNotEmpty()
+  signature!: string;
+}