readme: supported websites, benchmark results, llms.txt for discovery

The README now lists the tuned site shortcuts (Hacker News, Reddit,
DuckDuckGo, Bing) next to the generic engine, and cites the live
benchmark numbers from only-cli/benchmarks. llms.txt gives AI assistants
a one-page summary to index. npm keywords added for search.

The internal build spec moves out of the published repo.

package.json

@@ -20,9 +20,16 @@
   "keywords": [
     "cli",
     "ai-agents",
-    "web",
+    "web-browsing",
+    "token-efficient",
+    "claude",
+    "claude-code",
+    "chatgpt",
+    "codex",
+    "llm",
     "scraping",
-    "tokens"
+    "html-to-markdown",
+    "agent-tools"
   ],
   "author": "only-cli (https://github.com/only-cli)",
   "license": "MIT",