feat(M4-009,M4-010,M4-011): routing rules CRUD API, per-user overrides, agent capabilities

- Add routing.dto.ts with validation DTOs for create, update, and reorder operations
- Add routing.controller.ts with full CRUD: GET list, POST create, PATCH update,
  DELETE remove, PATCH reorder, GET effective (merged priority view)
- Users can only create/modify/delete their own user-scoped rules; system rules are
  protected with ForbiddenException
- GET /api/routing/rules/effective returns merged rule set with user rules taking
  precedence over system rules at the same priority level (M4-010)
- Extend agent-config.dto.ts with capability shorthand fields: domains, preferredModel,
  preferredProvider, toolSets (M4-011)
- Update agent-configs.controller.ts to merge capability fields into config.capabilities
  so agent's preferred model/provider can influence routing decisions
- Register RoutingController in agent.module.ts

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

apps/gateway/src/commands/command-executor.service.ts

@@ -138,30 +138,56 @@ export class CommandExecutorService {
     args: string | null,
     conversationId: string,
   ): Promise<SlashCommandResultPayload> {
-    if (!args) {
+    if (!args || args.trim().length === 0) {
+      // Show current override or usage hint
+      const currentOverride = this.chatGateway?.getModelOverride(conversationId);
+      if (currentOverride) {
+        return {
+          command: 'model',
+          conversationId,
+          success: true,
+          message: `Current model override: "${currentOverride}". Use /model <name> to change or /model clear to reset.`,
+        };
+      }
       return {
         command: 'model',
         conversationId,
         success: true,
-        message: 'Usage: /model <model-name>',
+        message:
+          'Usage: /model <model-name>  — sets a per-session model override (bypasses routing). Use /model clear to reset.',
       };
     }
-    // Update agent session model if session is active
-    // For now, acknowledge the request — full wiring done in P8-012
+
+    const modelName = args.trim();
+
+    // /model clear removes the override and re-enables automatic routing
+    if (modelName === 'clear') {
+      this.chatGateway?.setModelOverride(conversationId, null);
+      return {
+        command: 'model',
+        conversationId,
+        success: true,
+        message: 'Model override cleared. Automatic routing will be used for new sessions.',
+      };
+    }
+
+    // Set the sticky per-session override (M4-007)
+    this.chatGateway?.setModelOverride(conversationId, modelName);
+
     const session = this.agentService.getSession(conversationId);
     if (!session) {
       return {
         command: 'model',
         conversationId,
         success: true,
-        message: `Model switch to "${args}" requested. No active session for this conversation.`,
+        message: `Model override set to "${modelName}". Will apply when a new session starts for this conversation.`,
       };
     }
     return {
       command: 'model',
       conversationId,
       success: true,
-      message: `Model switch to "${args}" requested.`,
+      message: `Model override set to "${modelName}". The override is active for this conversation and will be used on the next message if a new session is needed.`,
     };
   }