feat(gateway,storage): mosaic gateway doctor with tier health JSON (FED-M1-06)

Adds `mosaic gateway doctor [--json] [--config <path>]` for tier-aware
health reporting (postgres, valkey, pgvector). JSON mode emits a single
parseable TierHealthReport for CI consumption; exit 1 on red.

Refactors gateway-internal tier probes into @mosaicstack/storage so the
same probe logic is reused by both the gateway boot path
(detectAndAssertTier — fail-fast on first failure) and the doctor
command (probeServiceHealth — non-throwing, runs all probes, returns
report). Old apps/gateway/src/bootstrap/tier-detector.* files removed.

A local TierConfig interface in tier-detection.ts captures the minimal
structural shape from MosaicConfig without importing @mosaicstack/config
(would create a cycle since config already depends on storage for
StorageConfig).

Tests:
- packages/storage/src/tier-detection.spec.ts — 22 tests (11 ported from
  gateway tier-detector, 11 new for probeServiceHealth + timing + branch
  coverage)
- packages/mosaic/src/commands/gateway-doctor.spec.ts — 12 tests for
  JSON contract, exit codes, output format, --config override

Refs #460

apps/gateway/src/main.ts

@@ -25,7 +25,7 @@ import { AppModule } from './app.module.js';
 import { mountAuthHandler } from './auth/auth.controller.js';
 import { mountMcpHandler } from './mcp/mcp.controller.js';
 import { McpService } from './mcp/mcp.service.js';
-import { detectAndAssertTier, TierDetectionError } from './bootstrap/tier-detector.js';
+import { detectAndAssertTier, TierDetectionError } from '@mosaicstack/storage';
 
 async function bootstrap(): Promise<void> {
   const logger = new Logger('Bootstrap');