refactor: complete pi-web rename in paths and env vars

- Config path: ~/.pi/pi-search/config.json -> ~/.pi/pi-web/config.json
- Env vars: PI_SEARCH_PROVIDER -> PI_WEB_PROVIDER,
  PI_SEARCH_SEARXNG_URL -> PI_WEB_SEARXNG_URL
- README + flake.nix description updated to match.

index.ts

@@ -1,6 +1,6 @@
 // Pi-Web Extension - Registers `web_search` and `web_fetch` tools backed by
 // a shared headless Firefox session. Provider config lives in
-// ~/.pi/pi-search/config.json (env overrides supported).
+// ~/.pi/pi-web/config.json (env overrides supported).
 import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
 import { Type } from "typebox";
 import { ConfigError, loadConfig, resolveSettings } from "./src/config.ts";