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/html_helpers.go

@@ -0,0 +1,58 @@
+package engines
+
+import (
+	"strings"
+)
+
+// extractAttr finds attr="value" or attr='value' in an HTML string.
+func extractAttr(s, attr string) string {
+	prefix := attr + `="`
+	idx := strings.Index(s, prefix)
+	if idx == -1 {
+		prefix = attr + "='"
+		idx = strings.Index(s, prefix)
+		if idx == -1 {
+			return ""
+		}
+	}
+	start := idx + len(prefix)
+	end := strings.Index(s[start:], "\"")
+	if end == -1 {
+		end = strings.Index(s[start:], "'")
+	}
+	if end == -1 {
+		end = len(s[start:])
+	}
+	return s[start : start+end]
+}
+
+// stripHTML removes all HTML tags from a string.
+func stripHTML(s string) string {
+	var result strings.Builder
+	inTag := false
+	for _, r := range s {
+		if r == '<' {
+			inTag = true
+			continue
+		}
+		if r == '>' {
+			inTag = false
+			continue
+		}
+		if !inTag {
+			result.WriteRune(r)
+		}
+	}
+	return strings.TrimSpace(result.String())
+}
+
+// htmlUnescape handles basic HTML entities.
+func htmlUnescape(s string) string {
+	s = strings.ReplaceAll(s, "&amp;", "&")
+	s = strings.ReplaceAll(s, "&lt;", "<")
+	s = strings.ReplaceAll(s, "&gt;", ">")
+	s = strings.ReplaceAll(s, "&quot;", "\"")
+	s = strings.ReplaceAll(s, "&#39;", "'")
+	s = strings.ReplaceAll(s, "&nbsp;", " ")
+	return s
+}