fix(research): keep Discuss chats grounded on their report (#4006)

* fix(research): preserve Discuss spin-off primer during context trimming

trim_for_context() kept only system_msgs[:1] as essential and dropped the
rest under budget pressure. A research "Discuss" spin-off seeds the report
as a system message that sits after the preface system messages, so it
landed in extra_system and was the first thing evicted once the chat grew
— the conversation then lost its grounding and drifted off task.

Treat any system message carrying research_spinoff_from metadata as
essential, alongside the leading system prompt, so the seeded report
survives trimming. maybe_compact already retains all system messages.

Tests: tests/test_context_compactor.py::TestResearchPrimerPreserved

* fix(research): ground Discuss spin-off chats on the seeded report

build_chat_context injected global memory (pinned + hybrid-retrieved) and
personal-doc RAG every turn, keyed off the user-level memory_enabled pref
and a request-scoped use_rag flag — never the session. A research spin-off,
whose primer declares the report the sole knowledge base, thus had
unrelated keyword-matched facts pulled in ("wrong data") competing with the
report; its rag=False flag was also ignored (use_rag defaulted on).

Add _session_is_research_spinoff(sess) (detects the primer research_spinoff_from
metadata; handles ChatMessage and dict forms) and, for such sessions,
disable memory injection and force RAG off.

Tests: tests/test_chat_helpers.py spin-off detection cases

---------

Co-authored-by: Dan (cirim) <claude@cirim.org>

tests/test_chat_helpers.py

@@ -218,3 +218,47 @@ def test_save_assistant_response_preserves_actual_and_requested_model():
 
     assert sess.history[-1].metadata["requested_model"] == "selected-model"
     assert sess.history[-1].metadata["model"] == "actual-model"
+
+
+from types import SimpleNamespace
+from routes.chat_helpers import _session_is_research_spinoff
+
+
+class _SpinMsg:
+    def __init__(self, role, metadata=None):
+        self.role = role
+        self.metadata = metadata
+
+
+def test_spinoff_detected_from_chatmessage_history():
+    sess = SimpleNamespace(history=[
+        _SpinMsg("system", {"research_spinoff_from": "rp-1"}),
+        _SpinMsg("user", None),
+    ])
+    assert _session_is_research_spinoff(sess) is True
+
+
+def test_spinoff_detected_from_dict_history():
+    sess = SimpleNamespace(history=[
+        {"role": "system", "metadata": {"research_spinoff_from": "rp-2"}},
+        {"role": "user", "content": "hi"},
+    ])
+    assert _session_is_research_spinoff(sess) is True
+
+
+def test_non_spinoff_plain_session_is_false():
+    sess = SimpleNamespace(history=[
+        _SpinMsg("system", {"compacted": True}),
+        _SpinMsg("user", None),
+    ])
+    assert _session_is_research_spinoff(sess) is False
+
+
+def test_metadata_on_non_system_message_ignored():
+    sess = SimpleNamespace(history=[_SpinMsg("user", {"research_spinoff_from": "rp-3"})])
+    assert _session_is_research_spinoff(sess) is False
+
+
+def test_empty_or_missing_history():
+    assert _session_is_research_spinoff(SimpleNamespace(history=[])) is False
+    assert _session_is_research_spinoff(SimpleNamespace()) is False