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/realism/test_email_prompt.py

@@ -0,0 +1,152 @@
+"""Prompt builder behaviour: language constraint, em-dash suppression,
+deterministic mannerism injection."""
+from __future__ import annotations
+
+import random
+
+from decnet.realism.personas import EmailPersona
+from decnet.realism.prompts.email import (
+    PromptInputs,
+    build,
+    select_mannerisms,
+)
+
+
+def _persona(**over) -> EmailPersona:
+    base = dict(
+        name="John Smith",
+        email="john@corp.com",
+        role="COO",
+        tone="formal",
+        mannerisms=[
+            "opens with 'I hope this finds you well'",
+            "uses 'Best regards' exclusively",
+            "references policy by number",
+            "ccs legal",
+        ],
+        language="en",
+    )
+    base.update(over)
+    return EmailPersona(**base)
+
+
+class _SeededRng:
+    """Adapter so prompt code thinks it has a SystemRandom."""
+
+    def __init__(self, seed: int):
+        self._r = random.Random(seed)
+
+    def shuffle(self, seq):
+        self._r.shuffle(seq)
+
+    def random(self):
+        return self._r.random()
+
+    def choice(self, seq):
+        return self._r.choice(seq)
+
+
+def test_select_mannerisms_returns_subset_of_pool():
+    persona = _persona()
+    picks = select_mannerisms(persona, rng=_SeededRng(0), n=2)
+    assert len(picks) == 2
+    assert all(m in persona.mannerisms for m in picks)
+
+
+def test_select_mannerisms_deterministic_under_same_seed():
+    persona = _persona()
+    a = select_mannerisms(persona, rng=_SeededRng(42), n=2)
+    b = select_mannerisms(persona, rng=_SeededRng(42), n=2)
+    assert a == b
+
+
+def test_select_mannerisms_returns_all_when_pool_smaller_than_n():
+    persona = _persona(mannerisms=["a"])
+    picks = select_mannerisms(persona, rng=_SeededRng(0), n=2)
+    assert picks == ["a"]
+
+
+def test_select_mannerisms_empty_pool():
+    persona = _persona(mannerisms=[])
+    assert select_mannerisms(persona) == []
+
+
+def test_build_includes_language_constraint_english():
+    sender = _persona(language="en")
+    recip = _persona(name="Sarah", email="sarah@corp.com", role="PM")
+    prompt, _ = build(
+        PromptInputs(sender=sender, recipient=recip, context_hint="budget"),
+        rng=_SeededRng(0),
+    )
+    assert "in English" in prompt
+
+
+def test_build_includes_language_constraint_spanish():
+    sender = _persona(language="es")
+    recip = _persona(name="Sarah", email="sarah@corp.com", role="PM")
+    prompt, _ = build(
+        PromptInputs(sender=sender, recipient=recip, context_hint="budget"),
+        rng=_SeededRng(0),
+    )
+    assert "in Spanish" in prompt
+
+
+def test_build_em_dash_suppression_default():
+    sender = _persona()
+    recip = _persona(name="Sarah", email="sarah@corp.com", role="PM")
+    prompt, _ = build(
+        PromptInputs(sender=sender, recipient=recip, context_hint="budget"),
+        rng=_SeededRng(0),
+    )
+    assert "Do NOT use em-dashes" in prompt
+
+
+def test_build_em_dash_lifted_for_llm_heavy_persona():
+    sender = _persona(uses_llms_heavily=True)
+    recip = _persona(name="Sarah", email="sarah@corp.com", role="PM")
+    prompt, _ = build(
+        PromptInputs(sender=sender, recipient=recip, context_hint="budget"),
+        rng=_SeededRng(0),
+    )
+    assert "Do NOT use em-dashes" not in prompt
+    assert "fine" in prompt.lower()
+
+
+def test_build_reply_thread_block_prefixes_re():
+    sender = _persona()
+    recip = _persona(name="Sarah", email="sarah@corp.com", role="PM")
+    prompt, _ = build(
+        PromptInputs(
+            sender=sender,
+            recipient=recip,
+            context_hint="budget",
+            parent_subject="Re: Q3 budget",
+            parent_excerpt="Numbers attached.",
+        ),
+        rng=_SeededRng(0),
+    )
+    assert "REPLY in an ongoing thread" in prompt
+    assert "Re: Q3 budget" in prompt
+    assert "Numbers attached" in prompt
+    assert "prefixed with 'Re: '" in prompt
+
+
+def test_build_returns_mannerisms_used_metadata():
+    sender = _persona()
+    recip = _persona(name="Sarah", email="sarah@corp.com", role="PM")
+    _, used = build(
+        PromptInputs(sender=sender, recipient=recip, context_hint="budget"),
+        rng=_SeededRng(7),
+    )
+    assert used
+    assert all(m in sender.mannerisms for m in used)
+
+
+def test_build_uses_explicit_signature_when_provided():
+    sender = _persona(signature="-- John\\nCOO")
+    recip = _persona(name="Sarah", email="sarah@corp.com", role="PM")
+    prompt, _ = build(
+        PromptInputs(sender=sender, recipient=recip, context_hint="budget"),
+        rng=_SeededRng(0),
+    )
+    assert "Use this exact signature block" in prompt