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/personas.py

@@ -0,0 +1,9 @@
+"""Persona schema — placeholder for stage 2.
+
+In stage 2 of the realism migration, this module receives the real
+persona schema currently living at
+``decnet.orchestrator.emailgen.personas`` (``EmailPersona``,
+``parse_personas``, ``in_active_hours``).  Stage 1 keeps it empty so
+the import path is reserved without behaviour.
+"""
+from __future__ import annotations