Move Forge release runner into CLI

[iancleary]

Summary

• move Forge's bump/cut release behavior into forge release built-ins
• add a generic builtin:cargo-release runner for single-root-Cargo-crate repos that should only need release.toml, with github and gitea providers
• point release.toml at the built-in runner and remove scripts/bump-version.sh / scripts/cut-release.sh
• update release docs, agent guidance, and the release-process check to use the CLI contract

Validation

• cargo fmt --check
• cargo check -p forge
• cargo run -q -p forge -- release check --json
• cargo run -q -p forge -- release plan --json
• cargo test -p forge
• just release-process-check
• just ci
• temp-repo smoke tests for generic builtin:cargo-release dry-run restore and GitHub/Gitea apply paths with stubbed provider CLIs