## Bug

config.rag.score_gate default 0.05 was incompatible with hybrid RRF
fusion_score: raw RRF tops out at num_retrievers / (k_rrf + 1) ≈
0.0328 at the default k_rrf=60, so every hybrid `kb ask` tripped
ScoreGate refusal even when the top hit was perfectly aligned across
both retrievers. Symptomatic on the post-P4-3 manual smoke at
/tmp/kb-smoke/ pointed at 192.168.0.47 Ollama:

    $ kb ask "Rust ownership 모델의 핵심 규칙은 뭐야?" --mode hybrid
    근거 부족. KB에 해당 내용 없음.        # top fusion_score = 0.0164

Per-mode score_gate (lexical_score_gate / vector_score_gate /
hybrid_score_gate) was rejected because it forces every consumer
(CLI, eval, TUI) to know which mode picks which threshold. Score
normalization solves it at the source.

## Fix

crates/kb-search/src/hybrid.rs divides every fused score by
2 / (k_rrf + 1), the theoretical RRF maximum with two retrievers
each contributing rank 1. After normalization:

- both retrievers agree on rank 1 → fusion_score = 1.0
- only one retriever finds the chunk → caps near 0.5
- typical mixed ranks → falls between 0 and 0.5

RRF's rank-ordering invariants are preserved (every score divides
by the same positive constant), so sort + tiebreak behaviour is
unchanged. Wire schema label `fusion_score` keeps its slot in
RetrievalDetail; only the magnitude shifts, and only for hybrid
mode (lexical / vector were already in [0, 1]).

Verification: re-ran the four-scenario smoke at /tmp/kb-smoke/ with
default score_gate = 0.05 — all four (Korean→Korean, English→
English, cross-language Korean↔English, out-of-corpus) succeed
with the expected grounded / refusal classification, top
fusion_score now ≈ 0.5.

## Tests

One unit test (rrf_formula_matches_known_value) updated to expect
the normalized value `(1/61 + 1/62) / (2/61) ≈ 0.9919` instead of
the raw `1/61 + 1/62 ≈ 0.0325`. The integration snapshot fixture
crates/kb-search/tests/fixtures/search/hybrid/run-1.json already
used presence checks (fusion_score_positive: true) rather than
absolute values, so it doesn't need regeneration. Workspace 319
tests pass; clippy clean across both feature configs.

## Docs

This commit also adds tasks/HOTFIXES.md as a dated post-merge log
covering this fix and the two earlier --config-flag regressions
(P3-5 hotfix #20 across ingest/search/list/inspect/doctor; P4-3
follow-up #24 for kb ask). Original task specs in tasks/p<N>/
*.md stay frozen as the historical contract; HOTFIXES.md is the
live source of truth for post-merge deltas. Each affected task
spec gets a "Risks/notes" addendum pointing back to HOTFIXES.md
so a reader landing on the spec sees the active behaviour:

- tasks/INDEX.md gains a "Post-merge 핫픽스" section.
- tasks/phase-3-vector-hybrid.md updates the RRF formula text to
  show the normalized form.
- tasks/p3/p3-4-hybrid-fusion.md "Behavior contract" RRF bullet
  notes the normalization and reason.
- tasks/p3/p3-5-app-wiring.md "Risks/notes" notes the --config
  fix.
- tasks/p4/p4-3-rag-pipeline.md "Risks/notes" notes the kb-ask
  --config fix and the score_gate-RRF incompatibility (closed by
  the normalization in p3-4).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

@@ -194,0 +195,4 @@
+        // Raw RRF: `Σ 1/(k_rrf + rank_m(c))` over the retrievers a chunk
+        // appears in. With two retrievers the raw upper bound is
+        // `2/(k_rrf + 1)` — at k_rrf=60 that's only ≈0.0328, which makes
+        // a single `config.rag.score_gate` default of 0.05 silently

@@ -194,2 +212,4 @@
         let FusionPolicy::Rrf { k_rrf } = self.fusion;
         let k_rrf_f = f64::from(k_rrf);
+        // Both retrievers can contribute, so the per-mode RRF max is
+        // 2 / (k_rrf + 1). Even when a chunk lands in only one mode, we

@@ -0,0 +1,71 @@
+---

@@ -83,2 +83,4 @@
   - [p9-5 desktop-tauri](p9/p9-5-desktop-tauri.md)
 
+## Post-merge 핫픽스
+

@@ -94,3 +94,3 @@
 - `SearchMode::Hybrid`:
   - run `lexical.search(query)` and `vector.search(query)` in sequence (fan-out is fine; not required).
-  - fuse with RRF: `score(c) = Σ_{m ∈ {lex, vec}} 1 / (k_rrf + rank_m(c))` where `k_rrf` from config (default 60). `rank_m` is 1-based; chunks not appearing in retriever `m` contribute 0.
+  - fuse with RRF: `raw(c) = Σ_{m ∈ {lex, vec}} 1 / (k_rrf + rank_m(c))` where `k_rrf` from config (default 60). `rank_m` is 1-based; chunks not appearing in retriever `m` contribute 0.

@@ -179,6 +179,8 @@ All tests under `cargo test -p kb-rag` with no real Ollama (mock LM only).
 ## Risks / notes
 
 - Citation regex is STRICT `\[#(\d{1,3})\]` only. Models that emit `[1]`/`[ #1 ]`/`[foo]` are treated as no-marker → refusal. This is intentional: a noisy citation grammar lets prose `[1]` or `vec![1]` slip through as false positives, which corrupts both `grounded` and `kb eval` `citation_coverage`. The prompt template (`rag-v1`) explicitly instructs `[#번호]`.
+- **Post-merge fix (2026-05)**: kb-cli's `Cmd::Ask` arm originally called bare `kb_app::ask(query, opts)`, ignoring `--config <path>` and silently using XDG defaults (manifested as wrong model id / score_gate / data_dir surfacing in `Answer.retrieval`). Fixed by routing through `kb_app::ask_with_config(cfg, query, opts)`. See [HOTFIXES.md](../HOTFIXES.md).