refactor(realism): move emailgen LLM/personas/prompt into shared library

Lift the format-agnostic pieces from decnet/orchestrator/emailgen/
into the new decnet/realism/ library so file-class content generation
(stage 3 of the realism migration) can reuse them. Email-specific
delivery (RFC 2822 EML, IMAP/POP3 spool, thread chains) stays in
orchestrator/.

Renames (history-preserving git mv):
  emailgen/personas.py     -> realism/personas.py
  emailgen/prompt.py       -> realism/prompts/email.py
  emailgen/global_pool.py  -> realism/personas_pool.py
  emailgen/llm/            -> realism/llm/

Env-var clean break (pre-v1, no aliases):
  DECNET_EMAILGEN_LLM      -> DECNET_REALISM_LLM
  DECNET_EMAILGEN_MODEL    -> DECNET_REALISM_MODEL
  DECNET_EMAILGEN_TIMEOUT  -> DECNET_REALISM_TIMEOUT
  DECNET_EMAILGEN_PERSONAS -> DECNET_REALISM_PERSONAS
  DECNET_EMAILGEN_FAKE_OUTPUT -> DECNET_REALISM_FAKE_OUTPUT

Importers rewritten in: orchestrator/emailgen/scheduler.py,
orchestrator/drivers/email.py, web/router/{emailgen,topology}/
api_personas.py, cli/emailgen.py. Tests for moved modules relocated
to tests/realism/; tests for stay-put modules updated in place.

API URL `/api/v1/emailgen/personas` and CLI `decnet emailgen
import-personas` keep their public names until the service-collapse
commit (stage 5).

decnet/realism/__init__.py

@@ -13,8 +13,8 @@ import from here.  This package owns:
   ``mtime`` sampler so planted files don't all stamp at wall-clock-now.
 * :mod:`decnet.realism.planner` — picks ``(decky, persona, class,
   action, mtime)`` tuples for the orchestrator's tick loop.
-* :mod:`decnet.realism.personas` — persona schema (lifted from
-  ``orchestrator.emailgen.personas`` in stage 2 of the migration).
+* :mod:`decnet.realism.personas` — persona schema (the
+  :class:`EmailPersona` record describing each fictional employee).
 * :mod:`decnet.realism.prompts` — prompt builders, one per content
   class, sharing an em-dash-suppression style helper.
 * :mod:`decnet.realism.llm` — :class:`LLMBackend` ABC + factory + impl