feat/simplify campaign builtin semantics

[zeroXbrock]

• remove "builtin:" prefixes from campaign defs
• remove unused code

Motivation

We don't really need to prefix builtin scenarios with builtin: in campaigns. Ending with .toml is enough to differentiate between builtin & file-based scenarios.

Solution

• remove prefixes from campaign files
  • I didn't remove the code that strips the builtin: prefix
  • this avoids breaking clients with outdated campaign files, and still works

PR Checklist

• Added Tests
• Added Documentation
• Ran cargo +nightly clippy --workspace --lib --examples --tests --benches --all-features --locked --fix
• Ran cargo fmt --all
• Note breaking changes in PR description, if applicable
• update changelogs
  • Update CHANGELOG.md in each affected crate
  • add a high-level description in the root changelog