[REFACTOR] Reorganize `src/chemgraph/models/` around protocol × endpoint

[InkedWings]

Problem

The modules under models/ look organized by API protocol, but the layout
mixes axes: openai.py / anthropic.py / gemini.py are protocol-shaped,
alcf_endpoints.py is gateway-shaped, groq.py is vendor-shaped, codex.py
is product-shaped.

Two consequences:

• ChatOpenAI is built in five places, two outside models/:
  openai.py:300/:317, alcf_endpoints.py:120, agent/turn.py:294,
  agent/llm_agent.py:350.
• Dispatch exists twice — the same if/elif chain in
  models/loader.py:77-113 and agent/llm_agent.py:298-336.

The root cause is that two independent dimensions — which protocol to speak,
and which endpoint to speak to — are collapsed into one model-name prefix.

Proposal

models/
  protocols/    openai_compatible.py  anthropic_native.py  google_native.py
  endpoints/    argo.py  alcf.py  openrouter.py  vllm.py
                openai_direct.py  anthropic_direct.py  google_direct.py
  groq.py  local_model.py  codex.py     # own clients, unchanged
  loader.py     # the only dispatch site

Endpoint specs declare only the differences — base URL, auth style,
model-name transform, per-model quirks, default params — and reuse the protocol
layer to build the client. One endpoint can then hold several specs (Argo two, ALCF three) and one protocol can back several endpoints.

Worth evaluating whether LLM Rosetta can provide the protocol layer here rather
than us maintaining our own wrappers.