feat(rag): pass chat history with prior thinking to the LLM

Phase 74 (TODO.md L4): a follow-up question now reaches the model WITH
the conversation so far — every prior user/brain turn and the prior
thinking blocks on brain turns (preserve-thinking) — while
POST /api/chat stays stateless (A10): the client provides the history
in the request body and the server stores nothing new.

Server (task 01):
- ChatRequest.history: optional list[HistoryTurn] (who: user|brain,
  text, optional thinking) — absent/empty keeps the request
  byte-identical to pre-phase-74 (the two-message [system, user]
  request; the kill-switch semantics are pinned in the integration
  suite).
- app.rag.prompts.history_to_messages: pure mapper — walks the turns
  newest-first against the settings budgets (history_max_turns=40 /
  history_max_chars=24000, BOR_HISTORY_MAX_TURNS /
  BOR_HISTORY_MAX_CHARS); a capped turn is dropped WHOLE (never cut
  mid-answer); the kept window is returned oldest-first; brain turns
  carry their thinking as reasoning_content (A4) only when
  non-empty.
- Both branches feed it: the deflected path splices it between the
  system prompt and the current user message (the phase-71 recovery
  still rebuilds from messages[1:]), the grounded agent receives
  run_agent(..., history=hist); llm.py's message params widen to
  list[dict[str, Any]] (string-only messages stay byte-identical on
  the wire — the SDK passes message dicts through verbatim).
- The per-turn log line (PLAN §9) gains history_msgs=N after
  kb_chars=N.
- Pins: tests/unit/test_history.py (mapper: mapping, reasoning
  gating, both budgets, drop-whole, ordering, empty default),
  tests/unit/test_config.py (the two settings + env overrides),
  tests/unit/test_agent.py (the history splice + the default),
  tests/integration/test_chat_api.py (deflected AND grounded forward
  the history incl. reasoning_content, no-history byte-identity, 422
  pins, the log field).

Client (task 02):
- runTurn — the single funnel for fresh send / phase-49 retry /
  phase-53 stale-regen — sends history = the conversation record
  minus the current question, with thinking only on brain records
  that streamed one (undefined drops the key from the JSON, the
  record's convention); the question is never duplicated into the
  history.

Wire proof (task 03):
- The mock's echo my history marker (HISTORY_TRIGGER) answers with
  the deterministic history echo — history: N prior messages; last
  answer tail: <last 24 chars>; thinking: yes|no — checked BEFORE
  the DEFLECT_MODE branch (like TABLE_TRIGGER), so it fires on both
  turn branches whatever the gate says; the module docstring records
  the user/assistant-only history invariant that keeps every
  existing (tool-result-classified) marker flow unaffected.
- tests/e2e/test_llm_history.py (isolated): a grounded follow-up and
  a deflected follow-up both receive history: 2 prior messages +
  thinking: yes + the byte-exact tail of turn 1's answer (derived
  from the persisted bor.chat.v1 record — the same array the client
  maps into the body); a cold start receives history: 0 prior
  messages / last answer tail: none / thinking: no.
- Regressions green in isolation: chat_rag, chat_history (phase 50),
  agent_document_tools, harness_aligned_tools, stop_generation,
  retry_answer, response_to_docs.

tests/unit/test_config.py

@@ -147,6 +147,48 @@ def test_llm_retry_delay_rejects_negative(monkeypatch: pytest.MonkeyPatch) -> No
         _settings()
 
 
+def test_history_budget_defaults(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Phase 74 (TODO L4): the client-provided history is trimmed
+    newest-first against the newest 40 turns within a total of
+    24 000 chars (text + prior thinking, owner-locked A3)."""
+    monkeypatch.delenv("BOR_HISTORY_MAX_TURNS", raising=False)
+    monkeypatch.delenv("BOR_HISTORY_MAX_CHARS", raising=False)
+    s = _settings()
+    assert s.history_max_turns == 40
+    assert s.history_max_chars == 24_000
+
+
+def test_history_budget_env_overrides(monkeypatch: pytest.MonkeyPatch) -> None:
+    """``BOR_HISTORY_MAX_TURNS`` / ``BOR_HISTORY_MAX_CHARS`` override the
+    defaults; ``0`` on either is the no-history kill switch (the
+    pre-phase-74 two-message requests)."""
+    monkeypatch.setenv("BOR_HISTORY_MAX_TURNS", "12")
+    monkeypatch.setenv("BOR_HISTORY_MAX_CHARS", "5000")
+    s = _settings()
+    assert s.history_max_turns == 12
+    assert s.history_max_chars == 5000
+    monkeypatch.setenv("BOR_HISTORY_MAX_TURNS", "0")
+    assert _settings().history_max_turns == 0
+    monkeypatch.setenv("BOR_HISTORY_MAX_CHARS", "0")
+    assert _settings().history_max_chars == 0
+
+
+def test_history_max_turns_rejects_negative(monkeypatch: pytest.MonkeyPatch) -> None:
+    """``0`` is the no-history kill switch — a negative value is a typo,
+    so the validator fails loudly at startup (the ``agent_max_rounds``
+    pattern)."""
+    monkeypatch.setenv("BOR_HISTORY_MAX_TURNS", "-1")
+    with pytest.raises(ValidationError, match="history_max_turns"):
+        _settings()
+
+
+def test_history_max_chars_rejects_negative(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A negative char budget is a typo — fail loudly at startup."""
+    monkeypatch.setenv("BOR_HISTORY_MAX_CHARS", "-1")
+    with pytest.raises(ValidationError, match="history_max_chars"):
+        _settings()
+
+
 def test_agent_max_rounds_default_and_env_override(monkeypatch: pytest.MonkeyPatch) -> None:
     """Phase 45: the per-tool budgets are gone — ``BOR_AGENT_MAX_ROUNDS``
     (default 10) is the single agent-loop knob; ``0`` is the no-tools