chore: add direct Mistral provider

[EwenQuim]

Summary

• Adds internal/providers/configs/mistral.json as a new first-class provider hitting https://api.mistral.ai/v1 (uses $MISTRAL_API_KEY)
• Registers it in providers.go alongside the other main providers
• 11 models included:
  • Mistral Medium 3.5 (default large, reasoning, 256k ctx, $1.5/$7.5/1M)
  • Magistral Medium 1.2 and Magistral Small (reasoning models, $2/$5 and $0.5/$1.5)
  • Mistral Large 3 ($0.5/$1.5)
  • Devstral 2 (agentic coding, 256k ctx, $0.4/$2) and Devstral Small 2
  • Codestral (code completion, 32k ctx, $0.3/$0.9)
  • Mistral Small 4 (default small, multimodal, $0.1/$0.3)
  • Ministral 14B / 8B / 3B (edge models)

Test plan

• go build ./... passes
• go test ./internal/providers/... — 37 tests pass
• Verify with a live $MISTRAL_API_KEY that model IDs resolve correctly

[EwenQuim]

Closing in favor of #332 which has a better implementation with a generator script and canonical -latest model IDs.