Make the default package intentionally small

[clifton]

Summary

• make the default install intentionally narrow: derive plus OpenAI
• keep Anthropic, Gemini, Grok, logging helpers, streaming, tools, mocks, and schemars opt-in
• add all-providers for applications that select a provider at runtime
• publish from an explicit source/docs/examples allowlist; the verified archive contains 73 files and compresses to 223.9 KiB
• build docs.rs with all features and lint default, schema-only, mock-only, schemars/mock, and every single-provider surface in CI
• align README, cookbook, integration-test gates, and doctests with the smaller defaults

Compatibility

Applications that previously relied on Anthropic, Gemini, Grok, or logging being enabled implicitly must now select those features. Runtime multi-provider applications can add features = ["all-providers"].

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• strict clippy across derive-only, mock-only, mock/schemars, derive/mock, and each derive/provider surface
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --no-fail-fast
• cargo test --doc
• cargo test --all-features --no-fail-fast
• cargo package --allow-dirty --target-dir tmp/pr5-package-target