feat(emailgen): Ollama-driven fake email worker for IMAP/POP3 deckies

Second orchestrator worker (decnet emailgen) that drips persona-driven,
threaded, multi-language fake emails into running mail deckies.  Personas
live on Topology.email_personas; topology-wide language_default falls
through to any persona that doesn't pin its own.  Em-dashes are
suppressed at the prompt layer by default and only lifted for personas
explicitly marked uses_llms_heavily — em-dashes are an LLM tell and a
flat corpus of em-dashed mail is a giveaway.

EML delivery writes into /var/spool/decnet-emails/<thread>/<msg>.eml on
the mail decky via docker exec; wiring the IMAP/POP3 templates to read
from that spool (replacing the hardcoded _BAIT_EMAILS) is the next step.

tests/orchestrator/emailgen/test_threads.py

@@ -0,0 +1,61 @@
+"""Thread-chain helpers."""
+from __future__ import annotations
+
+from decnet.orchestrator.emailgen.threads import (
+    ThreadChain,
+    new_message_id,
+    new_thread_id,
+    references_for_reply,
+    reply_subject,
+)
+
+
+def test_new_thread_id_is_uuid_string():
+    tid = new_thread_id()
+    assert len(tid) == 36
+    assert tid.count("-") == 4
+
+
+def test_new_message_id_format_with_domain():
+    mid = new_message_id("example.com")
+    assert mid.startswith("<") and mid.endswith(">")
+    assert "@example.com" in mid
+
+
+def test_new_message_id_handles_blank_domain():
+    mid = new_message_id("   ")
+    assert "@localhost" in mid
+
+
+def test_reply_subject_prepends_re():
+    assert reply_subject("Q3 budget") == "Re: Q3 budget"
+
+
+def test_reply_subject_collapses_existing_re():
+    assert reply_subject("Re: Re: Q3 budget") == "Re: Q3 budget"
+    assert reply_subject("RE: Q3 budget") == "Re: Q3 budget"
+
+
+def test_references_for_reply_root_is_empty():
+    assert references_for_reply(None) == ""
+
+
+def test_references_for_reply_appends_parent():
+    chain = ThreadChain(
+        thread_id="t1",
+        parent_message_id="<m2@x>",
+        references=("<m1@x>",),
+        parent_subject="Re: budget",
+    )
+    refs = references_for_reply(chain)
+    assert refs == "<m1@x> <m2@x>"
+
+
+def test_references_empty_chain_starts_with_parent_only():
+    chain = ThreadChain(
+        thread_id="t1",
+        parent_message_id="<m1@x>",
+        references=(),
+        parent_subject="budget",
+    )
+    assert references_for_reply(chain) == "<m1@x>"