ci: publish releases from public repository

[danielkov]

Summary

• publish tag-driven releases directly to speakeasy-api/kit with the repository GitHub token
• allow unsigned macOS artifacts when Apple credentials are absent, while retaining optional signing and notarization
• update installation/release documentation and retire the competing local publisher
• bump Kit to 0.1.83

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• OPENROUTER_API_KEY=ci-placeholder cargo test --locked
• cargo build --locked --release
• workflow YAML and embedded shell syntax checks
• release tag and optional signing branch checks