Adds FileCoveredByDocMap(cfg *DocMapConfig, file string) bool — a thin wrapper
over the existing unexported mappingMatches that lets cmd/ check per-file docmap
coverage without duplicating glob logic.
Also adds unit tests covering matched globs, non-matching paths, empty file,
and empty config.
- New --doc-map flag (DOC_MAP_FILE env var): path to YAML config mapping
source path globs to governing design docs
- New --doc-map-max-bytes flag (DOC_MAP_MAX_BYTES env var): cap on total
injected doc content, default 100KB
- review/docmap.go: DocMapConfig parsing, glob matching with ** support,
doc loading via VCS with directory expansion and size guard
- budget.Sections: new DesignDocs field, trimmed after conventions
- budget.buildResult: injects DesignDocs under ## Design Documents heading
- action.yml: doc-map and doc-map-max-bytes inputs wired to env vars
- CHANGELOG.md: created with unreleased entry
- Tests: ParseDocMapConfig, MatchDocs, globMatch, LoadMatchingDocs