refactor(rename): kb → kebab — binary, env vars, XDG paths, file renames

두 번째 commit. 사용자 facing surface (CLI binary, env vars, XDG paths)
+ 코드 안 single-letter token (`KB_`, `kb.sqlite`, `/kb/`, tracing
target) 일괄 rename. 그리고 3 개 file rename:

- 디자인 doc `2026-04-27-kb-final-form-design.md` →
  `2026-04-27-kebab-final-form-design.md`
- 최초 보고서 `kb_local_rust_report.md` → `kebab_local_rust_report.md`
- workspace ignore `.kbignore` → `.kebabignore`

## 변경

- `crates/kebab-cli/Cargo.toml`: `[[bin]] name = "kb"` → `"kebab"`.
- `crates/kebab-cli/src/main.rs`: `#[command(name = "kb", …)]` →
  `name = "kebab"`.
- 모든 `KB_*` env var (코드 + doc + 테스트) → `KEBAB_*`. apply_env
  prefix 매칭 + 30+ 개 setting 키 모두.
- XDG paths: `~/.config/kb` / `~/.local/share/kb` / `~/.cache/kb` /
  `~/.local/state/kb` → `~/.config/kebab` 등. config defaults +
  expand_path tests + paths.rs 의 hardcode 모두.
- SQLite filename: `kb.sqlite` → `kebab.sqlite` (`SQLITE_FILE` const
  + 테스트 hardcode 모두).
- tracing target: `target: "kb-*"` → `"kebab-*"` (10+ 곳).
- snapshot fixture: `.kbignore` → `.kebabignore` (`fixtures/source-fs/
  tree-1.snapshot.json` 갱신).

## 검증

- `cargo test --workspace -j 1` clean (linker OOM 회피 위해 직렬).
- `cargo clippy --workspace --all-targets -- -D warnings` clean.

다음 commit 에서 docs sweep.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

crates/kebab-search/tests/lexical.rs

@@ -29,7 +29,7 @@ impl Env {
         config.storage.data_dir = temp.path().to_string_lossy().into_owned();
         let store = SqliteStore::open(&config).expect("open store");
         store.run_migrations().expect("run migrations");
-        let db_path = temp.path().join("kb.sqlite");
+        let db_path = temp.path().join("kebab.sqlite");
         Self {
             _temp: temp,
             store: Arc::new(store),
@@ -618,7 +618,7 @@ fn lexical_snapshot_run_1() {
     // `Vec<SearchHit>` for a fixed query is checked verbatim against
     // `tests/fixtures/search/lexical/run-1.json`. Update both sides in
     // the same commit when intentional changes ship.
-    // Stable because rusqlite ships bundled SQLite — a tokenizer/bm25 algorithm change in a future SQLite bump will require regenerating run-1.json via `KB_UPDATE_SNAPSHOTS=1`.
+    // Stable because rusqlite ships bundled SQLite — a tokenizer/bm25 algorithm change in a future SQLite bump will require regenerating run-1.json via `KEBAB_UPDATE_SNAPSHOTS=1`.
     let env = Env::new();
     let conn = env.raw_conn();
     insert_document(&conn, &id32("d"), "notes/snap.md", "Snap", "en", "primary", &[]);
@@ -656,11 +656,11 @@ fn lexical_snapshot_run_1() {
 
     let baseline_path =
         std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/search/lexical/run-1.json");
-    if std::env::var_os("KB_UPDATE_SNAPSHOTS").is_some() {
+    if std::env::var_os("KEBAB_UPDATE_SNAPSHOTS").is_some() {
         std::fs::write(&baseline_path, serde_json::to_string_pretty(&actual).unwrap()).unwrap();
     }
     let baseline_text = std::fs::read_to_string(&baseline_path)
-        .expect("baseline snapshot must exist; run with KB_UPDATE_SNAPSHOTS=1 to seed");
+        .expect("baseline snapshot must exist; run with KEBAB_UPDATE_SNAPSHOTS=1 to seed");
     let expected: serde_json::Value = serde_json::from_str(&baseline_text).unwrap();
     assert_eq!(actual, expected, "lexical run-1 snapshot drift");
 }