Honor HTTP/HTTPS proxy env vars in Hub (#53)

[nyo16]

Closes #53

Problem

LlamaCppEx.Hub makes its HuggingFace requests through Req → Finch → Mint.
Unlike curl/wget/requests/Go, Mint does not read proxy environment
variables. On a proxy-only network every request goes direct and times out:

** (Req.TransportError) timeout
** (MatchError) no match of right hand side value: {:error, "network error: timeout"}

Fix

Add a proxy_request_options/2 resolver that builds Mint connect_options and
wire it into all four Req.get call sites (search, list_gguf_files,
get_model_info, and the streaming download).

A proxy is resolved from, in order:

1. The :proxy option — a URL string, a Mint {scheme, address, port, opts}
   tuple, or false to disable.
2. Standard environment variables — HTTPS_PROXY/HTTP_PROXY (and their
   lowercase forms) take precedence over ALL_PROXY, so a usable HTTP proxy
   always wins over an unusable SOCKS one.

Other behavior:

• NO_PROXY / :no_proxy host bypass — exact, .suffix, and *.
• Basic-auth from user:pass@ userinfo → proxy-authorization header
  (credentials redacted in logs).
• Since HuggingFace is HTTPS, the HTTPS_PROXY value applies and tunnels via
  the CONNECT method.

# honored automatically
export HTTPS_PROXY=http://127.0.0.1:8118

# or per call
LlamaCppEx.Hub.search("qwen3 gguf", proxy: "http://user:pass@127.0.0.1:8118")
LlamaCppEx.Hub.download("org/model", "model.gguf", proxy: false)

SOCKS is not supported (by design)

The underlying stack (Req → Finch → Mint) supports HTTP/1 proxies only —
plain forwarding and HTTPS-over-CONNECT. There is no SOCKS support anywhere in
Mint/Finch, so a socks5:// value (e.g. from ALL_PROXY) is detected and
skipped with an actionable warning. To use a SOCKS upstream, run a local
HTTP-to-SOCKS bridge (Privoxy, gost) and point HTTPS_PROXY at it. This is
documented in a new "Proxies" section in the module doc.

The original reporter's proxy is 127.0.0.1:8118 — Privoxy's default port —
i.e. they already run an HTTP→SOCKS bridge. With HTTPS_PROXY now honored and
winning over the SOCKS ALL_PROXY, their setup works as-is.

Tests

• 20 new TDD unit tests in test/hub_proxy_test.exs (env detection, precedence,
  NO_PROXY, basic-auth, SOCKS skip, default ports, scheme-less URLs).
• Full suite: 158 passed, 4 skipped, 7 excluded.
• mix compile --warnings-as-errors and mix format --check-formatted clean.