[codex] Add an intuitive extraction quickstart

[clifton]

Why

The structured-output entry point was named materialize, which is expressive once learned but does not immediately tell a new user that the method extracts a typed value from unstructured input. The first README example also referenced an old crate version and made broader reliability/provider claims than the public API can guarantee.

What changed

• adds LLMClient::extract as the primary descriptive structured-output entry point
• adds the same extract terminal to the fluent request builder
• keeps materialize fully supported as an equivalent, backwards-compatible name
• replaces the opening guide with a focused 60-second extraction path
• updates the installation snippet to the current crate version
• tightens provider and retry language so the README describes bounded, typed behavior precisely
• updates crate-level documentation to lead with extract

The alias delegates through the existing materialize contract, so third-party LLMClient implementations do not need to add a method and retain their existing routing behavior.

Verification

• cargo fmt --all -- --check
• cargo test --all-features --target-dir tmp/pr1-target
• cargo clippy --all-targets --all-features --target-dir tmp/pr1-target -- -D warnings
• regression tests cover direct custom-client delegation and request-builder system-context routing

The full run passed 299 library tests, all integration suites, and 83 doctests.