📝 docs: clarify release example contract

[zrr1999]

Summary

• Convert examples/hello.sp into a pure standalone example that does not rely on platform imports.
• Update README example contract from format/check/build to format/check/run for examples.
• Clarify direct imported platform API calls versus lower-level effect-handler syntax.
• Align CI with the documented examples contract by checking/running examples without native-building the project-mode foreign-function app.

Validation

• spore format --check examples/hello.sp && spore check examples/hello.sp && spore run examples/hello.sp
• cd examples/hello-app && spore format --check src/main.sp && spore check src/main.sp && spore run src/main.sp
• spore format --check && find src/basic_cli -name "*.sp" -type f | sort | xargs -n 1 spore check && spore check examples/hello.sp && spore check examples/hello-app/src/main.sp && find src/basic_cli -name "*.sp" -type f | sort | xargs -n 1 spore build
• uvx --from spore-lang==0.0.3 spore format --check plus CI-equivalent Spore check/build/run commands for platform modules and examples
• uvx --from spore-lang==0.0.3 spore test
• cargo fmt --check && cargo clippy --all-targets --all-features -- -D warnings && cargo test --all-targets --all-features

Notes

• Local spore is 0.0.2 and spore test fails with module basic_cli.stdout not found; CI-equivalent spore-lang==0.0.3 reports note: no spec clauses found and exits successfully.
• Native spore build remains covered for src/basic_cli platform API files; project-mode examples/hello-app is checked and run until backend support for foreign/platform lowering lands.