The resolver script needs a YAML parser but Node has no native YAML
support. Rather than requiring every downstream project to add `yaml`
as a dependency or teaching the installer to copy individual npm
packages, we vendor yaml@2.8.2 (zero transitive deps, ~800KB) into
src/scripts/vendor/yaml/ and import it via relative path.
- `_installSharedScripts` is now a dumb recursive copy of src/scripts/
into _bmad/scripts/ — no per-dependency logic.
- Future shared scripts can drop additional vendored modules into
src/scripts/vendor/<name>/ and require('./vendor/<name>') from
their source file.
- Lint/format ignore src/scripts/vendor/** so vendored code isn't
subject to project style rules.
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