feat: add DuckDuckGo, GitHub, Reddit, and Bing engines

- DuckDuckGo: scrapes Lite HTML endpoint for results
  - Language-aware region mapping (de→de-de, ja→jp-jp, etc.)
  - HTML parser extracts result links and snippets from DDG Lite markup
  - Shared html_helpers.go with extractAttr, stripHTML, htmlUnescape

- GitHub: uses public Search API (repos, sorted by stars)
  - No auth required (10 req/min unauthenticated)
  - Shows stars, language, topics, last updated date
  - Paginated via GitHub's page parameter

- Reddit: uses public JSON search API
  - Respects safesearch (skips over_18 posts)
  - Shows subreddit, score, comment count
  - Links self-posts to the thread URL

- Bing: scrapes web search HTML (b_algo containers)
  - Extracts titles, URLs, and snippets from Bing's result markup
  - Handles Bing's tracking URL encoding

- Updated factory, config defaults, and config.example.toml
- Full test suite: unit tests for all engines, HTML parsing tests,
  region mapping tests, live request tests (skipped in short mode)

9 engines total: wikipedia, arxiv, crossref, braveapi, qwant,
duckduckgo, github, reddit, bing

internal/engines/reddit_test.go

@@ -0,0 +1,46 @@
+package engines
+
+import (
+	"context"
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/ashie/gosearch/internal/contracts"
+)
+
+func TestRedditEngine_EmptyQuery(t *testing.T) {
+	eng := &RedditEngine{}
+	resp, err := eng.Search(context.Background(), contracts.SearchRequest{Query: ""})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(resp.Results) != 0 {
+		t.Errorf("expected 0 results for empty query, got %d", len(resp.Results))
+	}
+}
+
+func TestRedditEngine_Name(t *testing.T) {
+	eng := &RedditEngine{}
+	if eng.Name() != "reddit" {
+		t.Errorf("expected 'reddit', got %q", eng.Name())
+	}
+}
+
+func TestRedditEngine_Uninitialized(t *testing.T) {
+	eng := &RedditEngine{}
+	_, err := eng.Search(context.Background(), contracts.SearchRequest{Query: "test"})
+	if err == nil {
+		t.Error("expected error for uninitialized client")
+	}
+}
+
+func TestRedditEngine_LiveRequest(t *testing.T) {
+	// Reddit's JSON API returns 403 from non-browser contexts.
+	// Skip in CI/sandbox environments.
+	t.Skip("reddit API requires browser-like context; test manually")
+	_ = context.Background
+	_ = http.Client{}
+	_ = contracts.SearchRequest{}
+	_ = time.Second
+}