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_test.go

@@ -510,3 +510,63 @@ func TestFileCoveredByDocMap_EmptyConfig(t *testing.T) {
 		t.Error("expected false for empty config, got true")
 	}
 }
+
+// ============================================================
+// ParseDocMapConfigContent
+// ============================================================
+
+func TestParseDocMapConfigContent_Valid(t *testing.T) {
+	content := `
+mappings:
+  - paths:
+      - "lib/foo/**"
+    docs:
+      - docs/foo.md
+`
+	cfg, err := ParseDocMapConfigContent(content, "owner/repo@main:.review-bot/doc-map.yml")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(cfg.Mappings) != 1 {
+		t.Fatalf("expected 1 mapping, got %d", len(cfg.Mappings))
+	}
+	if len(cfg.Mappings[0].Docs) != 1 || cfg.Mappings[0].Docs[0] != "docs/foo.md" {
+		t.Errorf("unexpected mapping: %+v", cfg.Mappings[0])
+	}
+}
+
+func TestParseDocMapConfigContent_EmptyContent(t *testing.T) {
+	cfg, err := ParseDocMapConfigContent("", "test-source")
+	if err != nil {
+		t.Fatalf("unexpected error for empty content: %v", err)
+	}
+	if len(cfg.Mappings) != 0 {
+		t.Errorf("expected 0 mappings for empty content, got %d", len(cfg.Mappings))
+	}
+}
+
+func TestParseDocMapConfigContent_InvalidYAML(t *testing.T) {
+	_, err := ParseDocMapConfigContent("mappings: [{{invalid", "test-source")
+	if err == nil {
+		t.Fatal("expected error for invalid YAML, got nil")
+	}
+}
+
+func TestParseDocMapConfigContent_UnknownKeys(t *testing.T) {
+	content := `
+mappings:
+  - paths:
+      - "lib/**"
+    docs:
+      - docs/foo.md
+unknown_top_level_key: "should be warned but not fatal"
+`
+	// Unknown top-level keys produce a warning but not an error.
+	cfg, err := ParseDocMapConfigContent(content, "test-source")
+	if err != nil {
+		t.Fatalf("unexpected error for unknown keys: %v", err)
+	}
+	if len(cfg.Mappings) == 0 {
+		t.Error("expected mappings to be parsed despite unknown key")
+	}
+}