feat(#143): fetch doc-map config from trusted VCS ref

The doc-map YAML config was previously read from the local workspace
(the PR branch checkout). A malicious PR author could modify
.review-bot/doc-map.yml to map any path glob to sensitive design docs,
causing review-bot to fetch and inject those docs into the LLM prompt.

Fix: add --doc-map-trusted-ref (DOC_MAP_TRUSTED_REF) flag. When set to
a trusted ref (e.g. 'main'), the doc-map config is fetched from the VCS
API at that ref instead of from local workspace. A 404 from VCS is a
hard error (no silent fallback to local copy).

When unset, the local workspace is used with a security warning in the
logs pointing operators to the new flag.

Changes:
- review/docmap.go: add ParseDocMapConfigContent + parseDocMapBytes
  helper to parse from in-memory content (fetched via VCS API)
- cmd/review-bot/main.go: add --doc-map-trusted-ref flag; Step 6c
  branches on trusted-ref to fetch vs local-workspace load
- .gitea/actions/review/action.yml: add doc-map-trusted-ref input
- README.md: document new input
- CHANGELOG.md: security and feature entries

Tests:
- TestParseDocMapConfigContent_Valid/Empty/InvalidYAML/UnknownKeys
  in review/docmap_test.go

Coverage: 53.0% cmd/review-bot

cmd/review-bot/main.go

@@ -101,6 +101,7 @@ func main() {
 	aicoreResourceGroup := flag.String("aicore-resource-group", envOrDefault("AICORE_RESOURCE_GROUP", "default"), "SAP AI Core resource group (for provider=aicore)")
 	docMapFile := flag.String("doc-map", envOrDefault("DOC_MAP_FILE", ""), "Path to YAML file mapping source path globs to governing design docs")
 	docMapMaxBytes := flag.Int("doc-map-max-bytes", envOrDefaultInt("DOC_MAP_MAX_BYTES", review.DefaultDocMapMaxBytes), "Maximum bytes of injected doc content (default 102400)")
+	docMapTrustedRef := flag.String("doc-map-trusted-ref", envOrDefault("DOC_MAP_TRUSTED_REF", ""), "Git ref (e.g. main) to fetch the doc-map config from via VCS API instead of local workspace. Recommended to prevent PR branch from controlling which docs are injected.")
 
 	flag.Parse()
 
@@ -368,10 +369,45 @@ func main() {
 	// Step 6c: Load path-scoped design docs if doc-map specified
 	designDocs := ""
 	if *docMapFile != "" {
-		docMapCfg, err := review.ParseDocMapConfig(resolvedDocMapFile)
-		if err != nil {
-			slog.Error("failed to parse doc-map file", "file", *docMapFile, "error", err)
-			os.Exit(1)
+		var docMapCfg *review.DocMapConfig
+
+		if *docMapTrustedRef != "" {
+			// Fetch doc-map config from a trusted VCS ref (e.g. the default branch).
+			// This prevents a malicious PR from modifying the doc-map config to
+			// inject arbitrary docs into the LLM prompt.
+			slog.Info("doc-map: fetching config from trusted ref",
+				"path", *docMapFile,
+				"ref", *docMapTrustedRef)
+			content, fetchErr := vcs.GetFileContentRef(ctx, owner, repoName, *docMapFile, *docMapTrustedRef)
+			if fetchErr != nil {
+				slog.Error("doc-map: failed to fetch config from trusted ref",
+					"path", *docMapFile,
+					"ref", *docMapTrustedRef,
+					"error", fetchErr)
+				os.Exit(1)
+			}
+			source := fmt.Sprintf("%s/%s@%s:%s", owner, repoName, *docMapTrustedRef, *docMapFile)
+			var parseErr error
+			docMapCfg, parseErr = review.ParseDocMapConfigContent(content, source)
+			if parseErr != nil {
+				slog.Error("doc-map: failed to parse fetched config",
+					"source", source,
+					"error", parseErr)
+				os.Exit(1)
+			}
+		} else {
+			// Local workspace fallback — the doc-map is read from the PR branch checkout.
+			// SECURITY WARNING: a malicious PR can modify this file to inject arbitrary
+			// docs. Set --doc-map-trusted-ref (or DOC_MAP_TRUSTED_REF) to a trusted ref
+			// (e.g. "main") to fetch the config from the default branch instead.
+			slog.Warn("doc-map: loading config from local workspace (PR branch) — " +
+				"set --doc-map-trusted-ref to fetch from a trusted ref for security")
+			var parseErr error
+			docMapCfg, parseErr = review.ParseDocMapConfig(resolvedDocMapFile)
+			if parseErr != nil {
+				slog.Error("failed to parse doc-map file", "file", *docMapFile, "error", parseErr)
+				os.Exit(1)
+			}
 		}
 
 		// Collect changed file paths from the PR for intersection.
@@ -385,10 +421,11 @@ func main() {
 
 		if len(matchedDocs) > 0 {
 			docMapOpts := review.DocMapOptions{MaxBytes: *docMapMaxBytes}
-			designDocs, err = review.LoadMatchingDocs(ctx, vcs, owner, repoName, matchedDocs, docMapOpts)
-			if err != nil {
+			var loadErr error
+			designDocs, loadErr = review.LoadMatchingDocs(ctx, vcs, owner, repoName, matchedDocs, docMapOpts)
+			if loadErr != nil {
 				// Non-fatal: individual missing files are already warned; log and continue.
-				slog.Warn("doc-map: partial failure loading docs", "error", err)
+				slog.Warn("doc-map: partial failure loading docs", "error", loadErr)
 			}
 			if designDocs != "" {
 				slog.Info("doc-map: injected design docs", "matched", len(matchedDocs), "bytes", len(designDocs))