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.

.agents/phases/complete/73_hidden_tab_stream/00_phase.md

@@ -0,0 +1,28 @@
+# Phase 73 — Hidden tab never stops a generating answer
+
+**Source:** `TODO.md` L3 — "Clicking on another tab while an answer is generating stops that answer from being generated. Reponses should continue to generate unless you outright close the tab."
+**Story:** n/a (TODO-derived)
+**Context:** `frontend/assets/app.js` (the SSE turn machine: `runTurn` ~L1859, the `pagehide` partial-persist handler ~L2162, the 120s pre-token guard `TURN_TIMEOUT_MS` ~L287 / `armTurnTimeout` ~L982, `readSSE` ~L1044, the settle paths `done` ~L2011 / stop ~L2079), `app/api/chat.py` (the `finally` "turn cancelled" log line — the server-side signal that the SSE consumer really went away), phase-48 teardown contract (a REAL consumer departure — tab closed, navigation, Stop — still cancels the fetch and stops the model: that behavior is correct and must survive this phase).
+
+## Objective
+An in-flight answer keeps generating while the browser tab is merely hidden (switched away from) and completes when the user returns; only closing the tab, navigating away, or clicking Stop aborts the turn. Also fixes the latent record-corruption on that path (a `pagehide` partial persist can leave a duplicated brain turn in the saved conversation, which makes the answer *look* truncated on restore).
+
+## Dependencies
+— (none)
+
+## Tasks
+1. `01_repro_root_cause.md` — bounded repro with an instrumentation decision tree; pin down WHICH mechanism stops the answer on tab switch (no permanent code changes).
+2. `02_fix_hidden_tab.md` — the fix: correlate the pagehide partial with the turn's settle so `done`/stop *replaces* it (never appends a second brain turn); if the repro implicates the 120s guard, make hidden time not count toward it; keep phase-48 teardown for real departures.
+3. `03_e2e_hidden_tab_stream.md` — Playwright regression: synthetic `pagehide` mid-stream → the answer completes exactly once, the record has one brain turn, reload restores it; regressions + commit.
+
+## Testing & Quality
+- Unit/integration: frontend-only phase — no `app/` changes expected (coverage floor unaffected, must stay **>90%** on `app/`).
+- E2E: new story suite `tests/e2e/test_hidden_tab_stream.py`, run in isolation (`uv run pytest tests/e2e/test_hidden_tab_stream.py -v --no-cov`) against the deterministic mock LLM (long/slow deterministic streams give a guaranteed mid-stream window).
+- Regression runs in isolation: `test_chat_rag.py`, `test_chat_persistence.py`, `test_chat_history.py` (phase 50), `test_stop_generation.py` (phase 48 contract: real Stop/cancel still tears down), `test_retry_answer.py`.
+
+## Completion Criteria
+- [ ] The repro's root cause is named in the phase-73 commit message body (one line: which candidate from task 01 fired, or "none of C1–C3 — <finding>").
+- [ ] `uv run pytest tests/e2e/test_hidden_tab_stream.py -v --no-cov` green in isolation: a tab switch (synthetic `pagehide`) mid-turn never stops the answer, and the persisted conversation holds exactly one brain turn for that question.
+- [ ] Real departures unchanged: Stop button, tab close, and navigation still cancel the fetch (phase-48 `test_stop_generation.py` + `test_chat_persistence.py` green).
+- [ ] `uv run pytest` green; `uv run pytest --cov=app --cov-report=term-missing` >90%; `uv run ruff check . && uv run pyright` clean.
+- [ ] One atomic `--no-gpg-sign` Conventional-Commits commit (e.g. `fix(chat): keep generating while the tab is hidden`); phase dir moved to `.agents/phases/complete/`.