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

review/docmap.go

@@ -52,15 +52,31 @@ func ParseDocMapConfig(localPath string) (*DocMapConfig, error) {
 	if err != nil {
 		return nil, fmt.Errorf("read doc-map file %q: %w", localPath, err)
 	}
+	return parseDocMapBytes(data, localPath)
+}
 
+// 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>").
+//
+// Use this when the config content has been fetched from a trusted VCS ref
+// rather than read from the local workspace.
+func ParseDocMapConfigContent(content, source string) (*DocMapConfig, error) {
+	data := []byte(content)
+	return parseDocMapBytes(data, source)
+}
+
+// parseDocMapBytes is the shared YAML parse implementation used by
+// ParseDocMapConfig and ParseDocMapConfigContent.
+func parseDocMapBytes(data []byte, source string) (*DocMapConfig, error) {
 	var cfg DocMapConfig
 	if err := yaml.UnmarshalWithOptions(data, &cfg, yaml.Strict()); err != nil {
 		// Re-parse without strict mode to log which keys are unknown.
 		var relaxed DocMapConfig
 		if err2 := yaml.Unmarshal(data, &relaxed); err2 != nil {
-			return nil, fmt.Errorf("parse doc-map YAML %q: %w", localPath, err)
+			return nil, fmt.Errorf("parse doc-map YAML %q: %w", source, err)
 		}
-		slog.Warn("doc-map YAML contains unknown keys (ignored)", "file", localPath, "error", err)
+		slog.Warn("doc-map YAML contains unknown keys (ignored)", "file", source, "error", err)
 		cfg = relaxed
 	}
 	return &cfg, nil