feat(kebab-app + kebab-cli): p9-fb-18 CLI ask --session multi-turn

도그푸딩 item 14 — CLI 에서도 multi-turn 가능하도록 `kebab ask
--session <id>` 추가. p9-fb-17 의 ChatSessionRepo 위에 build, 첫 호출
세션 자동 생성, 이후 호출이 prior turns 를 history 로 받아 follow-up.
external AI integration (Claude Code skill / MCP) 도 같은 facade 로
stateful 대화 가능.

## 핵심 변경

- **`App::ask_with_session(session_id, query, opts) -> Answer`** —
  load session header → list_turns 로 prior history → 빌드 retriever
  stack (lexical / vector / hybrid 같은 분기) → `RagPipeline::ask_
  with_history` 호출 → 첫 호출이면 `chat_sessions` row 자동 생성
  (title = first_question_title) → `chat_turns` 새 row append.
- **`App::first_question_title(question)`** helper — `trim() + nfc()
  + 40 chars cap`, fallback `"untitled"`. unicode-normalization
  workspace dep 재사용.
- **`App::blake3_truncate(input)`** helper — `blake3(session_id ||
  ":" || turn_index)` 의 첫 16 byte 를 u128 으로, format!{:032x} 로
  32-hex `turn_id`.
- **`ask_with_session_with_config`** facade — CLI 진입점.
- **CLI `--session <id>` flag** — `Cmd::Ask` 의 `session: Option<
  String>` field, handler 가 None 이면 `ask_with_config` (기존
  단발), Some(id) 면 `ask_with_session_with_config` 호출.
- **에러 정책**: session create / turn append 실패 시 warn 로그
  남기고 answer 는 그대로 반환 — 사용자가 답변 받은 컴퓨트를 잃지
  않음. 영속성 실패가 답변 응답을 가로막지 않는 conservative shape.

## 테스트

- `App::first_question_title` 3 unit (trim + cap, empty → untitled,
  korean NFD → NFC)
- `App::blake3_truncate` 1 unit (deterministic + distinct across
  varying session/index)
- 워크스페이스 전체 `cargo test --workspace --no-fail-fast -j 1` exit 0
- `cargo clippy --workspace --all-targets -- -D warnings` clean

## 문서

- README `kebab ask` 행: `--session` 안내 + chat_sessions 자동 생성
  + `kebab reset --data-only` wipe 안내
- README **외부 AI 통합** 절: Claude Code skill 이 `--session` 으로
  multi-turn 가능하다는 한 문장 추가
- HANDOFF entry
- spec status planned → in_progress

## Out of scope (spec deviation)

- `--repl` (stdin loop) — spec 명시되어 있으나 stdin fixture 부담
  으로 deferral. 별도 후속 task 또는 `--session` 사용자 경험 회신
  후 결정.
- session list / show / delete 관리 명령 (spec 의 Out of scope).

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

crates/kebab-cli/src/main.rs

@@ -124,6 +124,16 @@ enum Cmd {
         /// to another tool that doesn't want trailing metadata.
         #[arg(long)]
         hide_citations: bool,
+
+        /// p9-fb-18: persistent multi-turn chat session id. First call
+        /// auto-creates the session in SQLite (`chat_sessions`), each
+        /// subsequent call with the same id loads prior turns as
+        /// history and appends the new Q/A. Without this flag, ask
+        /// is single-shot (no persistence). The session id is
+        /// caller-supplied — pick anything stable per conversation
+        /// (e.g. `kb-rust-async-2026-05`).
+        #[arg(long, value_name = "ID")]
+        session: Option<String>,
     },
 
     /// Wipe XDG data dirs (and optionally the Lance vector store) so the
@@ -453,6 +463,7 @@ fn run(cli: &Cli) -> anyhow::Result<()> {
             seed,
             show_citations,
             hide_citations,
+            session,
         } => {
             let cfg = kebab_config::Config::load(cli.config.as_deref())?;
             let opts = kebab_app::AskOpts {
@@ -465,14 +476,19 @@ fn run(cli: &Cli) -> anyhow::Result<()> {
                 // once on completion). The TUI ask pane (P9-3) is what
                 // wires up a real `mpsc::Sender` here.
                 stream_sink: None,
-                // p9-fb-15: CLI single-shot ask. p9-fb-18 adds
-                // `--session` / `--repl` for multi-turn over the same
-                // facade (passes a populated `history`).
+                // p9-fb-18: when `--session` is set, the facade
+                // (`ask_with_session_with_config`) loads prior turns
+                // from SQLite and stuffs them into AskOpts.history
+                // before calling `ask_with_history`. Single-shot path
+                // (no `--session`) keeps the empty defaults.
                 history: Vec::new(),
                 conversation_id: None,
                 turn_index: None,
             };
-            let ans = kebab_app::ask_with_config(cfg, query, opts)?;
+            let ans = match session.as_deref() {
+                Some(sid) => kebab_app::ask_with_session_with_config(cfg, sid, query, opts)?,
+                None => kebab_app::ask_with_config(cfg, query, opts)?,
+            };
             if cli.json {
                 println!("{}", serde_json::to_string(&wire::wire_answer(&ans))?);
             } else {