fix(consumer-prices): disable migros_ch, coop_ch, sainsburys_gb (#2113)

All three return 0/12 every run:
- migros_ch, coop_ch: Exa domain bleed (competitor URLs returned)
- sainsburys_gb: Exa returns no pages for all GB grocery queries

Disabled with dated comments explaining the root cause.

consumer-prices-core/configs/retailers/coop_ch.yaml

@@ -5,7 +5,8 @@ retailer:
   currencyCode: CHF
   adapter: search
   baseUrl: https://www.coop.ch
-  enabled: true
+  # disabled 2026-03: Exa returns competitor domain URLs (domain bleed) for CH queries; 0/12 every run.
+  enabled: false
 
   searchConfig:
     numResults: 5

consumer-prices-core/configs/retailers/migros_ch.yaml

@@ -5,7 +5,8 @@ retailer:
   currencyCode: CHF
   adapter: search
   baseUrl: https://www.migros.ch
-  enabled: true
+  # disabled 2026-03: Exa returns competitor domain URLs (domain bleed) for all CH queries; 0/12 every run.
+  enabled: false
 
   searchConfig:
     numResults: 5

consumer-prices-core/configs/retailers/sainsburys_gb.yaml

@@ -5,7 +5,8 @@ retailer:
   currencyCode: GBP
   adapter: search
   baseUrl: https://groceries.sainsburys.co.uk
-  enabled: true
+  # disabled 2026-03: Exa returns "no pages found" for all GB grocery queries; 0/12 every run.
+  enabled: false
 
   searchConfig:
     numResults: 5