feat: add provider-configuration skill for provider auth and credential chains

[AdamTylerLynch]

Adds a new provider-configuration skill to terraform-provider-development, filling
the plugin's largest gap: nothing currently covers a provider's Configure() method,
provider schema for authentication, or credential resolution — the first thing every
provider author has to build and the first thing every provider user hits.

What the skill teaches

• Provider schema for auth: all auth attributes Optional (never Required, so env
  fallback works), secrets Sensitive: true, descriptions naming the env-var fallback
• The credential provider chain pattern: a one-method Provider interface, a
  sentinel ErrNoCredentials distinguishing "nothing to offer, fall through" from
  "source misconfigured, surface it", and an aggregate ChainError that lists every
  source tried — with canonical precedence static config > environment variables >
  shared credentials file (profiles) > platform identity
• Resolving secrets as a complete set (never mixing an env key with a file secret)
  while resolving non-secret settings field-by-field
• Unknown-value guards in Configure(), secret redaction via String()/GoString(),
  credentials-file permission warnings, and eager configure-time validation with a
  skip_credentials_validation escape hatch
• Unit testing the chain hermetically (injectable env getter, t.TempDir() fixtures)

Files

• skills/provider-configuration/SKILL.md (317 lines)
• skills/provider-configuration/references/credential-chain.md — a complete,
  self-contained implementation for a fictional examplecloud provider
• skills/provider-configuration/references/case-studies.md — the
  hashicorp/aws-sdk-go-base chain (static short-circuit → SDK default chain →
  web-identity override → assume-role wrapping, NoValidCredentialSourcesError,
  GetCallerIdentity validation) and a generalized small-provider hand-rolled design
• Registers the skill in terraform/README.md, AGENTS.md, CHANGELOG.md

Verification

• All Go code blocks in references/credential-chain.md were extracted into a scratch
  module: go build ./..., go vet ./..., and go test ./... all pass (precedence,
  aggregation, hard-error, and redaction tests included)
• scripts/validate-structure.sh passes