[dead-code] chore: remove obsolete modelsdev catalog pricing API - #48739

This change removes dead pkg/modelsdev pricing-catalog code and the tests/specs that only exercised that API surface. It keeps the normalization helpers still used by CLI cost logic and updates package-local docs to match the reduced public surface.

• API surface reduction (pkg/modelsdev)

  • Removed unused pricing-catalog functions from catalog.go:
    • FindPricing
    • ensureCatalog
    • downloadAndParseCatalog
    • parseCatalog
    • parseCostMap
  • NormalizeProvider and NormalizeComparableModelID remain as the package’s active public API.

• Test/spec cleanup

  • Deleted pkg/modelsdev/catalog_test.go (tests tied to removed catalog/pricing internals).
  • Removed TestSpec_PublicAPI_FindPricing and TestSpec_DesignDecision_ProviderAliases from pkg/modelsdev/spec_test.go.
  • Kept normalization-focused spec coverage.

• Documentation alignment

  • Updated pkg/modelsdev/README.md to remove pricing lookup references/examples and describe the normalization-only API.

provider := modelsdev.NormalizeProvider(" github_models ")
modelID := modelsdev.NormalizeComparableModelID(" GPT_4.1-mini ")
// provider == "github-copilot"
// modelID == "gpt-4-1-mini"

• Fixes [dead-code] chore: remove dead functions — 5 functions removed #48660