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/prompts/__init__.py

@@ -1,7 +1,9 @@
 """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).
+* :mod:`decnet.realism.prompts.email` — corporate-email body builder.
+
+Stage 6 of the realism migration adds ``filebody.py``, ``filename.py``,
+and a ``_style.py`` helper so em-dash suppression sits in one place
+across email + file-class prompts.
 """
 from __future__ import annotations

decnet/realism/prompts/email.py

@@ -0,0 +1,154 @@
+"""Prompt builder for the email content class.
+
+The LLM gets a tightly-scoped instruction and a small handful of
+deterministic constraints.  Persona mannerisms are *pre-selected* in
+Python (1–2 of the persona's full list) and injected as hard rules —
+small models otherwise treat the mannerism list as flavour text and
+ignore it, and the corpus collapses into one voice.
+
+**Em-dash suppression** is on by default; suppression is lifted only
+for personas that opt in via ``uses_llms_heavily``.  Em-dashes are a
+strong stylometric tell for LLM-authored prose, and a honeypot mailbox
+where every author uses them is a tell.  Stage 6 of the realism
+migration extracts the suppression block into a shared
+``decnet.realism.prompts._style`` helper so file-class prompts pick
+it up too.
+"""
+from __future__ import annotations
+
+import secrets
+from dataclasses import dataclass
+from typing import Optional
+
+from decnet.realism.personas import EmailPersona
+
+
+@dataclass(frozen=True)
+class PromptInputs:
+    sender: EmailPersona
+    recipient: EmailPersona
+    context_hint: str
+    parent_subject: Optional[str] = None      # set when replying
+    parent_excerpt: Optional[str] = None      # short snippet of last msg
+
+
+_LANGUAGE_NAMES = {
+    "en": "English",
+    "es": "Spanish",
+    "pt": "Portuguese",
+    "fr": "French",
+    "de": "German",
+    "it": "Italian",
+    "nl": "Dutch",
+    "ja": "Japanese",
+    "zh": "Chinese",
+}
+
+
+def _lang_label(code: str) -> str:
+    return _LANGUAGE_NAMES.get(code.lower(), code)
+
+
+def select_mannerisms(
+    persona: EmailPersona,
+    *,
+    rng: Optional[secrets.SystemRandom] = None,
+    n: int = 2,
+) -> list[str]:
+    """Pick *n* mannerisms deterministically given *rng*.
+
+    Returns up to *n*; falls back to the full list when the persona
+    declares fewer.  Determinism (under a seeded RNG) is what makes
+    tests practical — otherwise mannerism injection is unverifiable.
+    """
+    rnd = rng or secrets.SystemRandom()
+    pool = list(persona.mannerisms)
+    if not pool:
+        return []
+    if len(pool) <= n:
+        return pool
+    rnd.shuffle(pool)
+    return pool[:n]
+
+
+def build(
+    inputs: PromptInputs,
+    *,
+    rng: Optional[secrets.SystemRandom] = None,
+) -> tuple[str, list[str]]:
+    """Return ``(prompt, mannerisms_used)``.
+
+    ``mannerisms_used`` flows back into the persisted ``payload`` JSON
+    so an analyst can see *why* a given email reads the way it does.
+    """
+    sender = inputs.sender
+    recipient = inputs.recipient
+    language = _lang_label(sender.language or "en")
+    mannerisms = select_mannerisms(sender, rng=rng)
+    mannerism_block = (
+        "\n".join(f"- {m}" for m in mannerisms)
+        if mannerisms
+        else "- (no specific mannerisms; write in the persona's tone)"
+    )
+
+    if sender.uses_llms_heavily:
+        em_dash_rule = (
+            "Em-dashes are fine — this persona uses them naturally. "
+            "Write in your usual style."
+        )
+    else:
+        em_dash_rule = (
+            "Do NOT use em-dashes (—). Use commas, periods, or "
+            "parentheses instead. Em-dashes are a tell."
+        )
+
+    sig_block = (
+        f"Use this exact signature block:\n{sender.signature}"
+        if sender.signature
+        else "End with a short, plausible signature for the persona's role."
+    )
+
+    if inputs.parent_subject:
+        thread_block = (
+            f"This is a REPLY in an ongoing thread.\n"
+            f"- Parent subject: {inputs.parent_subject}\n"
+            f"- Parent excerpt: {inputs.parent_excerpt or '(no excerpt)'}\n"
+            f"- Begin the body assuming the recipient already read the parent.\n"
+        )
+        subject_rule = (
+            "Subject must be the parent subject prefixed with 'Re: ' "
+            "(no double 'Re: Re:')."
+        )
+    else:
+        thread_block = "This is a NEW thread (no prior context)."
+        subject_rule = (
+            "Generate a short, specific subject line (≤ 80 chars) "
+            "appropriate to the context."
+        )
+
+    prompt = f"""You are writing one corporate email, RFC 2822 plain-text body only.
+
+Persona — sender:
+- Name: {sender.name}
+- Role: {sender.role}
+- Tone: {sender.tone_custom if sender.tone == "custom" and sender.tone_custom else sender.tone}
+- Mannerisms (must show through):
+{mannerism_block}
+
+Persona — recipient:
+- Name: {recipient.name}
+- Role: {recipient.role}
+
+Context hint: {inputs.context_hint}
+
+Thread context:
+{thread_block}
+
+Hard rules:
+1. Write the email body in {language}. Do not translate or code-switch.
+2. {em_dash_rule}
+3. {subject_rule}
+4. {sig_block}
+5. Output ONLY the email — first line is "Subject: <subject>", then a blank line, then the body. No commentary, no markdown fences, no preamble.
+"""
+    return prompt.strip(), mannerisms