new-terraform-provider: add provider.go scaffold with Configure and auth guidance

[AdamTylerLynch]

The scaffolding skill stopped at main.go — which imports internal/provider and
calls provider.New(version), a package the skill never helped create. This adds the
missing half.

Changes

• New assets/provider.go: provider struct and New(version) factory; Metadata;
  Schema with endpoint (Optional) and api_key (Optional + Sensitive,
  description naming the DEMO_API_KEY env fallback); Configure with unknown-value
  guards, env-var fallback, and actionable missing-value diagnostics naming both the
  attribute and its env var; empty Resources/DataSources registries — all with
  TODO comments in the same style as the existing assets/main.go
• SKILL.md gains the provider.go step and a sharper trigger description, and closes
  with soft pointers to the provider-configuration skill (growing the env fallback
  into a full credential provider chain) and provider-resources (first resource)

Verification: followed the skill's own steps in a scratch module —
go mod init → go get terraform-plugin-framework → go mod tidy →
go build -o /dev/null → go vet → go test ./... — all pass with the two assets
as written. scripts/validate-structure.sh passes.