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).

tests/orchestrator/emailgen/test_driver.py

@@ -5,10 +5,10 @@ from __future__ import annotations
 import pytest
 
 from decnet.orchestrator.drivers import email as email_driver
-from decnet.orchestrator.emailgen.llm.base import LLMResult, LLMTimeout
-from decnet.orchestrator.emailgen.llm.impl.fake import FakeBackend
-from decnet.orchestrator.emailgen.personas import EmailPersona
 from decnet.orchestrator.emailgen.scheduler import EmailAction
+from decnet.realism.llm.base import LLMResult, LLMTimeout
+from decnet.realism.llm.impl.fake import FakeBackend
+from decnet.realism.personas import EmailPersona
 
 
 class _RaisingBackend: