feat: agent routing engine — cost/capability matrix (P2-003)

Add RoutingService that selects optimal LLM model based on task type,
cost tier, capability requirements (reasoning, image, context window),
and provider preference. Scoring algorithm ranks all available models.

- RoutingService: route() for best match, rank() for scored list
- POST /api/providers/route and /api/providers/rank endpoints
- Routing types in @mosaic/types (CostTier, TaskType, RoutingCriteria)
- Cost tier classification: cheap (<$1/M), standard (<$10/M), premium

Closes #21

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

apps/gateway/src/agent/providers.controller.ts

@@ -1,11 +1,16 @@
-import { Controller, Get, UseGuards } from '@nestjs/common';
+import { Body, Controller, Get, Post, UseGuards } from '@nestjs/common';
+import type { RoutingCriteria } from '@mosaic/types';
 import { AuthGuard } from '../auth/auth.guard.js';
 import { ProviderService } from './provider.service.js';
+import { RoutingService } from './routing.service.js';
 
 @Controller('api/providers')
 @UseGuards(AuthGuard)
 export class ProvidersController {
-  constructor(private readonly providerService: ProviderService) {}
+  constructor(
+    private readonly providerService: ProviderService,
+    private readonly routingService: RoutingService,
+  ) {}
 
   @Get()
   list() {
@@ -16,4 +21,14 @@ export class ProvidersController {
   listModels() {
     return this.providerService.listAvailableModels();
   }
+
+  @Post('route')
+  route(@Body() criteria: RoutingCriteria) {
+    return this.routingService.route(criteria);
+  }
+
+  @Post('rank')
+  rank(@Body() criteria: RoutingCriteria) {
+    return this.routingService.rank(criteria);
+  }
 }