fix(#143): skip local doc-map validation when --doc-map-trusted-ref is set

When --doc-map-trusted-ref is provided, the --doc-map value is used as a
VCS API path parameter, not a local filesystem path. The early call to
validateWorkspacePath (which requires the file to exist locally) blocked
the trusted-ref code path when the doc-map did not exist in the local
checkout — defeating the feature's purpose in sparse checkouts or when
the file is only on the default branch.

Fix: guard the early validation with `&& *docMapTrustedRef == ""`.

Also fixes:
- review/docmap.go: correct ParseDocMapConfigContent godoc example to
  match actual source format "owner/repo@ref:path"
- cmd/review-bot/main_test.go: add TestMainSubprocess_DocMapTrustedRefSkipsLocalValidation
  to prevent regression

review/docmap.go

@@ -57,7 +57,7 @@ func ParseDocMapConfig(localPath string) (*DocMapConfig, error) {
 
 // ParseDocMapConfigContent parses a doc-map YAML config from an in-memory
 // string. The source parameter is used only for error messages and log entries
-// (e.g. "main:main@<ref>").
+// (e.g. "owner/repo@main:.review-bot/doc-map.yml").
 //
 // Use this when the config content has been fetched from a trusted VCS ref
 // rather than read from the local workspace.