Add forge release contract

[iancleary]

Summary

• add forge release check|plan|run backed by repo-local release.toml
• add Forge's own release.toml pointing at the existing just cut-release runner
• add the managed release-runner skill and docs for agent use

Validation

• cargo fmt -- --check
• cargo clippy -- -D warnings
• cargo test
• just install-list-check && just release-process-check
• cargo run -p forge -- --json release check
• cargo run -p forge -- --json release plan