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/duckduckgo.go

@@ -0,0 +1,87 @@
+package engines
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"github.com/ashie/gosearch/internal/contracts"
+)
+
+// DuckDuckGoEngine searches DuckDuckGo's Lite/HTML endpoint.
+// DuckDuckGo Lite returns a simple HTML page that can be scraped for results.
+type DuckDuckGoEngine struct {
+	client *http.Client
+}
+
+func (e *DuckDuckGoEngine) Name() string { return "duckduckgo" }
+
+func (e *DuckDuckGoEngine) Search(ctx context.Context, req contracts.SearchRequest) (contracts.SearchResponse, error) {
+	if strings.TrimSpace(req.Query) == "" {
+		return contracts.SearchResponse{Query: req.Query}, nil
+	}
+	if e == nil || e.client == nil {
+		return contracts.SearchResponse{}, errors.New("duckduckgo engine not initialized")
+	}
+
+	endpoint := fmt.Sprintf(
+		"https://lite.duckduckgo.com/lite/?q=%s&kl=%s",
+		url.QueryEscape(req.Query),
+		duckduckgoRegion(req.Language),
+	)
+
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
+	if err != nil {
+		return contracts.SearchResponse{}, err
+	}
+	httpReq.Header.Set("User-Agent", "gosearch/0.1 (compatible; +https://git.ashisgreat.xyz/penal-colony/gosearch)")
+
+	resp, err := e.client.Do(httpReq)
+	if err != nil {
+		return contracts.SearchResponse{}, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
+		return contracts.SearchResponse{}, fmt.Errorf("duckduckgo upstream error: status=%d body=%q", resp.StatusCode, string(body))
+	}
+
+	results, err := parseDuckDuckGoHTML(resp.Body)
+	if err != nil {
+		return contracts.SearchResponse{}, err
+	}
+
+	return contracts.SearchResponse{
+		Query:               req.Query,
+		NumberOfResults:    len(results),
+		Results:             results,
+		Answers:             []map[string]any{},
+		Corrections:         []string{},
+		Infoboxes:           []map[string]any{},
+		Suggestions:         []string{},
+		UnresponsiveEngines: [][2]string{},
+	}, nil
+}
+
+// duckduckgoRegion maps language codes to DDG region parameters.
+func duckduckgoRegion(lang string) string {
+	lang = strings.ToLower(strings.TrimSpace(lang))
+	if lang == "" || lang == "auto" {
+		return "us-en"
+	}
+	langCode := strings.SplitN(lang, "-", 2)[0]
+	regionMap := map[string]string{
+		"en": "us-en", "de": "de-de", "fr": "fr-fr", "es": "es-es",
+		"pt": "br-pt", "ru": "ru-ru", "ja": "jp-jp", "zh": "cn-zh",
+		"ko": "kr-kr", "it": "it-it", "nl": "nl-nl", "pl": "pl-pl",
+	}
+	if region, ok := regionMap[langCode]; ok {
+		return region
+	}
+	return "wt-wt"
+}