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/prompts/__init__.py

@@ -0,0 +1,7 @@
+"""Prompt builders for LLM-enriched content.
+
+Populated in stage 2 (``email.py`` lifted from
+``orchestrator.emailgen.prompt``) and stage 6 (``filebody.py``,
+``filename.py``, ``_style.py`` for em-dash suppression).
+"""
+from __future__ import annotations