refactor(emailgen): pluggable LLM backend (base/factory/impl)

Lift the Ollama subprocess shell-out out of EmailDriver and into a
proper provider subpackage shape:

  decnet/orchestrator/emailgen/llm/
    base.py        — LLMBackend Protocol + LLMResult + LLMTimeout
    factory.py     — get_llm() reads DECNET_EMAILGEN_LLM
    impl/ollama.py — current 'ollama run' subprocess path
    impl/fake.py   — canned-output backend used by tests

Driver now takes an LLMBackend on construction (or inherits the
factory default).  Tests inject FakeBackend instead of monkeypatching
the subprocess layer, which is cleaner and ~10x faster.  Swapping
Ollama for the Anthropic API / vLLM / llama.cpp is now a third branch
in factory.py; no driver rewrite needed.

Mirrors the convention used by decnet.web.db.factory + decnet.bus.factory
per the provider-subpackages-from-day-one rule in memory.

decnet/orchestrator/emailgen/llm/factory.py

@@ -0,0 +1,46 @@
+"""Backend dispatch.
+
+Reads ``DECNET_EMAILGEN_LLM`` to pick a concrete :class:`LLMBackend`.
+Defaults to ``ollama`` because that's what the prototype proved out and
+what most dev boxes have on hand.
+
+Supported keys:
+
+* ``ollama`` — :class:`decnet.orchestrator.emailgen.llm.impl.ollama.OllamaBackend`
+* ``fake``   — :class:`decnet.orchestrator.emailgen.llm.impl.fake.FakeBackend`
+  (canned output, used by tests so they don't shell out)
+
+Anthropic / vLLM / llama.cpp slots in here as a third branch when the
+need shows up.  Per the provider-subpackages memory, do NOT collapse
+factory dispatch into the impl modules — keeps the ``__init__`` import
+graph cycle-free and the env contract auditable in one place.
+"""
+from __future__ import annotations
+
+import os
+from typing import Any
+
+from decnet.orchestrator.emailgen.llm.base import LLMBackend
+
+
+def get_llm(*, model: str | None = None, **kwargs: Any) -> LLMBackend:
+    """Instantiate the LLM backend selected by environment.
+
+    *model* (when provided) overrides whatever the backend's own default
+    is — e.g. for OllamaBackend that's ``llama3.1`` unless
+    ``DECNET_EMAILGEN_MODEL`` says otherwise.  Lets the worker honour
+    ``decnet emailgen run --model gpt-oss`` without each backend having
+    to know about CLI flags.
+    """
+    backend_key = os.environ.get("DECNET_EMAILGEN_LLM", "ollama").lower()
+
+    if backend_key == "ollama":
+        from decnet.orchestrator.emailgen.llm.impl.ollama import OllamaBackend
+        return OllamaBackend(model=model, **kwargs)
+    if backend_key == "fake":
+        from decnet.orchestrator.emailgen.llm.impl.fake import FakeBackend
+        return FakeBackend(model=model or "fake-model", **kwargs)
+    raise ValueError(
+        f"Unsupported DECNET_EMAILGEN_LLM={backend_key!r}; "
+        "expected one of: ollama, fake"
+    )