feat(v0.17.0/A5): trigram-aware build_match_string + SearchResponse.hint

PR-A 본체. plan Task A4 Step 1c + A5.

- lexical.rs::build_match_string 재설계: whole-phrase + token-AND
  OR-combined, 3자 미만 토큰 drop, 후보 없음 시 None (빈 MATCH
  회피). raw single-quote mode 유지.
- SearchResponse.hint additive — empty result + trimmed < 3 chars
  + non-raw 케이스에 short_query_hint helper 가 set.
- CLI 'kebab search' 가 [hint] stderr 한 줄 (text mode).
- TUI SearchState.short_query_hint + poll_worker stale-aware set
  + fire_search/mark_input_changed reset + dynamic_status 표시.
- docs/wire-schema/v1/search_response.schema.json hint additive.
- 신규 unit tests (lexical 9 PASS, 기존 2 expectation 갱신) +
  통합 회귀 (search_korean: multi_token + mixed, 3 PASS) +
  BM25 snapshot regen (trigram token stream).

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

crates/kebab-app/src/app.rs

@@ -73,6 +73,37 @@ pub struct SearchResponse {
     /// p9-fb-37: present when caller passed `SearchOpts.trace = true`.
     /// Consumers that ignore trace should leave this `None`.
     pub trace: Option<kebab_core::SearchTrace>,
+    /// v0.17.0 A5 Step 4b: human / agent-readable advisory string set
+    /// when the empty hit list is likely due to a query shorter than the
+    /// FTS5 trigram tokenizer's 3-char minimum. `None` otherwise. CLI
+    /// surfaces it on stderr (text mode); MCP / `--json` consumers
+    /// surface it however they prefer. See
+    /// `docs/superpowers/specs/2026-05-22-korean-trigram-tokenizer-design.md`
+    /// §3.3.
+    pub hint: Option<String>,
+}
+
+/// v0.17.0 A5 Step 4b: decide whether to attach a "3자 이상 키워드 권장"
+/// hint to a `SearchResponse`. Fires only when the result set is empty
+/// *and* the trimmed query is shorter than the trigram tokenizer can
+/// resolve. Raw FTS5 mode (`'...'`) opts out — the user explicitly
+/// invoked FTS5 syntax. Identical condition powers the CLI stderr line
+/// and (separately) the TUI status bar.
+pub fn short_query_hint(query_text: &str, hits_empty: bool) -> Option<String> {
+    if !hits_empty {
+        return None;
+    }
+    let trimmed = query_text.trim();
+    let bytes = trimmed.as_bytes();
+    // Raw single-quote mode: user opted into FTS5 syntax, no advisory.
+    if bytes.len() >= 2 && bytes[0] == b'\'' && bytes[bytes.len() - 1] == b'\'' {
+        return None;
+    }
+    if trimmed.chars().count() < 3 {
+        Some("3자 이상 키워드 권장 (trigram tokenizer 제약)".to_string())
+    } else {
+        None
+    }
 }
 
 /// Facade state — see module docs for lifetime rules.
@@ -418,11 +449,13 @@ impl App {
 
             // Trace path skips the budget loop. Caller will inspect
             // `hits.len()` and `trace.timing` rather than paginate.
+            let hint = short_query_hint(&query.text, hits.is_empty());
             return Ok(SearchResponse {
                 hits,
                 next_cursor: None,
                 truncated: false,
                 trace: Some(trace),
+                hint,
             });
         }
 
@@ -505,11 +538,13 @@ impl App {
             None
         };
 
+        let hint = short_query_hint(&query.text, hits.is_empty());
         Ok(SearchResponse {
             hits,
             next_cursor,
             truncated,
             trace: None,
+            hint,
         })
     }