Add a config-driven, strongly-typed secret/env resolution system to
notsecrets with 11 provider backends. The system supports explicit
key bindings with typed SecretRef enums and a priority chain fallback.
Tier 1 providers (implemented): Env, 1Password, dotenvx, SOPS, GSM
Tier 2 providers (stubbed): nuenv, direnv, mise, Bitwarden, Vault, dotenvy
New types: Provider, ProviderConfig, SecretRef, SecretsConfig,
SecretsError, SecretSource, EnumerableSecretSource, SecretResolver
Config lives in notsecrets.toml with typed serde deserialization.
ISP split: EnumerableSecretSource for backends that support bulk
enumeration, SecretSource for single-key lookup only.
Includes conformance test suite, 33 new tests (161 workspace total).
notstrap EnvInjector migration deferred to follow-up.
