feat(02-02): add Mistral, Inflection, AI21 provider YAMLs

- 3 Tier 1 low-confidence providers with keyword anchoring
- Dual-located in providers/ and pkg/providers/definitions/
- Tier 1 total now at 12/12 providers

pkg/providers/definitions/mistral.yaml

@@ -0,0 +1,22 @@
+format_version: 1
+name: mistral
+display_name: Mistral AI
+tier: 1
+last_verified: "2026-04-05"
+keywords:
+  - "mistral"
+  - "mistral.ai"
+  - "MISTRAL_API_KEY"
+  - "la_plateforme"
+  - "api.mistral.ai"
+patterns:
+  - regex: '[a-zA-Z0-9]{32}'
+    entropy_min: 4.0
+    confidence: low
+verify:
+  method: GET
+  url: https://api.mistral.ai/v1/models
+  headers:
+    Authorization: "Bearer {KEY}"
+  valid_status: [200]
+  invalid_status: [401, 403]