feat(realism): scaffold decnet/realism/ library

Empty subpackage skeleton for the realism migration: ContentClass enum
(file/email/canary content categories), Plan dataclass (frozen, with
edit-action invariant), in_work_hours window check (wrap-around
supported, fail-open on parse error), and sample_mtime for backdated
file timestamps that snap into a persona's active hours.

Stage 1 of the orchestrator+canary realism unification — no
production caller wired yet; planner.pick is a stub returning None
until stage 3.

decnet/realism/llm/__init__.py

@@ -0,0 +1,8 @@
+"""LLM backend ABC + factory + impls.
+
+Populated in stage 2 of the realism migration: lifts the existing
+``orchestrator.emailgen.llm`` subpackage as-is (``base``, ``factory``,
+``impl/ollama``, ``impl/fake``).  Stage 6 adds ``circuit.py`` for
+cross-call breaker behaviour.
+"""
+from __future__ import annotations