The YAML AST/parser/emitter/edit-resolve module is removed. The
substrate now supports md / jsonc / jsonl. Walker, universal verbs,
CLI, and tests are stripped of yaml-kind branches; the `yaml` package
dependency is dropped.
Why: YAML editing was the most complex per-kind module (~750 LoC of
parse/emit/resolve plus walker scaffold) for the smallest surface area
in real-world usage — substrate consumers (lkg, gateway config, agent
metadata) all pivoted to jsonc / md / jsonl. Carrying yaml support
indefinitely was net cost.
Walker depth-cap test that previously relied on YAML's lack of a
parser-level depth cap is rewritten to construct a synthetic JSONC AST
chain by hand, exercising the walker's MAX_TRAVERSAL_DEPTH defense in
isolation from the parser's MAX_PARSE_DEPTH.
Net: -1467 LoC across substrate + tests.
