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-cli/src/main.rs

@@ -933,6 +933,15 @@ fn run(cli: &Cli) -> anyhow::Result<()> {
                     let next = resp.next_cursor.as_deref().unwrap_or("(none)");
                     eprintln!("[truncated; use --cursor {next} for the next page]");
                 }
+                // v0.17.0 A5 Step 4: short-query advisory. `resp.hint`
+                // is `Some` only when the result list is empty and the
+                // trimmed query is shorter than the trigram tokenizer
+                // can resolve (raw FTS5 mode opts out). stderr so it
+                // doesn't pollute the stdout hit list. `--json` skips
+                // this branch entirely; the field rides the wire.
+                if let Some(hint) = &resp.hint {
+                    eprintln!("[hint] {hint}");
+                }
                 if *trace {
                     if let Some(t) = &resp.trace {
                         eprintln!();