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-eval/tests/runner.rs

@@ -7,7 +7,7 @@
 //!   workspace's source-of-truth,
 //! - lexical-only retrieval (`SearchMode::Lexical`) so no embedder is
 //!   required (`models.embedding.provider = "none"`),
-//! - golden YAML pointed at via `KB_EVAL_GOLDEN`.
+//! - golden YAML pointed at via `KEBAB_EVAL_GOLDEN`.
 //!
 //! Determinism: lexical-only with a fixed seed corpus produces
 //! byte-identical `per_query.jsonl` content (modulo `run_id` /
@@ -24,7 +24,7 @@ use kebab_store_sqlite::SqliteStore;
 use rusqlite::params;
 use tempfile::TempDir;
 
-/// `KB_EVAL_GOLDEN` is process-global state. Tests touching it must
+/// `KEBAB_EVAL_GOLDEN` is process-global state. Tests touching it must
 /// serialize so they don't trample each other when `cargo test`
 /// runs them in parallel.
 static GOLDEN_ENV_LOCK: Mutex<()> = Mutex::new(());
@@ -143,19 +143,19 @@ fn lexical_opts() -> EvalRunOpts {
     }
 }
 
-/// Run the eval after pointing `KB_EVAL_GOLDEN` at `yaml`. The env
+/// Run the eval after pointing `KEBAB_EVAL_GOLDEN` at `yaml`. The env
 /// guard must outlive the call so concurrent tests don't reset the
 /// var mid-run.
 fn run_with_golden<F: FnOnce() -> R, R>(yaml: &Path, f: F) -> R {
     let _g = GOLDEN_ENV_LOCK.lock().unwrap_or_else(|p| p.into_inner());
-    // SAFETY: `KB_EVAL_GOLDEN` is a benign env var; the GOLDEN_ENV_LOCK
+    // SAFETY: `KEBAB_EVAL_GOLDEN` is a benign env var; the GOLDEN_ENV_LOCK
     // serializes mutations so concurrent tests don't race.
     unsafe {
-        std::env::set_var("KB_EVAL_GOLDEN", yaml);
+        std::env::set_var("KEBAB_EVAL_GOLDEN", yaml);
     }
     let out = f();
     unsafe {
-        std::env::remove_var("KB_EVAL_GOLDEN");
+        std::env::remove_var("KEBAB_EVAL_GOLDEN");
     }
     out
 }