Improve end-user onboarding and release install

[swoelffel]

Summary

• add interactive llmsh setup for provider/API-key/model onboarding
• add macOS/Linux release installer with checksum verification and setup handoff
• add GitHub Releases workflow for prebuilt binaries
• update README and project docs, including public Git flow guidance

Verification

cargo fmt --all -- --check
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace --locked
sh scripts/test-install.sh
sh -n install.sh scripts/test-install.sh
git diff --check README.md docs install.sh scripts/test-install.sh crates/llmsh-cli/src/main.rs crates/llmsh-cli/Cargo.toml crates/llmsh-core/src/setup.rs crates/llmsh-core/src/config/persist.rs .github/workflows/release.yml Cargo.lock

Notes

• Release artifacts and the ubuntu-24.04-arm GitHub runner will be validated by the first tagged release workflow run.
• The installer test suite covers mocked release downloads, checksums, PATH guidance, macOS post-copy handling, and TTY setup fallback.