phase: 112_honesty_gate_weak_hits

**Phase 112 — final verification pass (all 4 tasks already complete in `complete/`):**

- Verified gate fix: `app/api/chat.py::plan_turn` — HIGH iff `best_cosine >= relevance_threshold` OR (`fts_hits > 0` AND `best_cosine >= lexical_support_floor`); `lexical_support_floor` (default 0.35, `BOR_LEXICAL_SUPPORT_FLOOR`, bounds-validated) in `app/config.py` + `.env.example`; A8 revision note (2026-09-14) in `.agents/PLAN.md`.
- Verified prompt contract: `app/rag/prompts.py` diff is docstring-only (dated owner-decision-iii entry); `tests/unit/test_prompt_lock.py` byte-pins PERSONA/TOOLS_SECTION/DEFLECT body (sha256+length).
- Verified README: L11 + L575 deflection copy refreshed; `grep "haven't done anything" README.md` → no hits; disclosed-answer behavior documented.
- Tests: `uv run pytest --cov=app --cov-report=term-missing` → **2378 passed, 99% coverage (>90%)**; includes Mongolia-quadrant unit pins (fts>0 + cosine<floor → LOW).
- E2E in isolation: `uv run pytest tests/e2e/test_honest_deflection.py -v --no-cov` → **4 passed** (out-of-KB question: `deflected=true`, `sources==[]`, 2–3 suggestions); regression `test_chat_rag.py` + `test_retrieval_quality.py` → **7 passed**.
- Lint/types: `uv run ruff check .` → clean; `uv run pyright` → 0 errors.

**Completion criteria:** weak-FTS→LOW unit-pinned ✅ · no false citations + 2–3 alternatives E2E ✅ · prompts byte-identical (test-pinned) + README matches ✅ · suite/coverage/e2e/lint all green ✅ · commit + phase move → left to harness (no `git commit` run, per rules; changes in working tree).

**Deviations:** none. Next pending phase: `113_source_chip_quality`.

app/config.py

@@ -117,6 +117,15 @@ class Settings(BaseSettings):
     # default never discriminated. LOW only fires when best cosine < this
     # AND no candidate chunk matches the question lexically (see A8).
     relevance_threshold: float = 0.62
+    #: Lexical support floor (A8 revised 2026-09-14): an FTS hit promotes
+    # a turn to HIGH only when the best cosine is >= this value — it
+    # requires the vector signal to corroborate the lexical match. A
+    # single weak token hit with vector-unsupported docs (cosine < floor)
+    # stays LOW (deflected). Default 0.35 ≈ half the relevance threshold;
+    # tunable via ``BOR_LEXICAL_SUPPORT_FLOOR``. Must be <=
+    # ``relevance_threshold`` (a floor above the threshold is a typo that
+    # would make every FTS hit require a HIGH cosine anyway).
+    lexical_support_floor: float = 0.35
     #: Maximum output tokens a chat answer may use (owner instruction
     #: 2026-08-22: answers must run to their natural end — the old hard
     #: 700-token cap cut long answers off mid-sentence).
@@ -302,6 +311,22 @@ class Settings(BaseSettings):
     #: separate from ``sources_dir`` (the source checkouts).
     docs_work_dir: str = "~/bor-docs"
 
+    @field_validator("lexical_support_floor")
+    @classmethod
+    def _lexical_support_floor_bounds(cls, v: float, info: ValidationInfo) -> float:
+        """The lexical support floor must be in [0, relevance_threshold].
+        A value above the relevance threshold would be a typo — it would
+        make every FTS hit require a HIGH cosine anyway, defeating the
+        purpose of the floor (A8 revised 2026-09-14)."""
+        if v < 0:
+            raise ValueError("lexical_support_floor must be >= 0")
+        threshold = info.data.get("relevance_threshold")
+        if isinstance(threshold, float) and v > threshold:
+            raise ValueError(
+                f"lexical_support_floor ({v}) must be <= relevance_threshold ({threshold})"
+            )
+        return v
+
     @field_validator("import_extensions")
     @classmethod
     def _import_extensions_known(cls, v: str) -> str: