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/metrics_and_compare.rs

@@ -34,7 +34,7 @@ fn cfg_with_data_dir(tmp: &TempDir, golden_yaml: &str) -> Config {
     // SAFELY scoped — `set_var` is process-global so callers serialise
     // tests via the `serial_test`-style guard below.
     unsafe {
-        std::env::set_var("KB_EVAL_GOLDEN", &golden_path);
+        std::env::set_var("KEBAB_EVAL_GOLDEN", &golden_path);
     }
     cfg
 }
@@ -127,9 +127,9 @@ fn write_run(
     store.record_eval_run_with_results(&row, &results).unwrap();
 }
 
-/// Each test mutates a process-global env var (`KB_EVAL_GOLDEN`) and
+/// Each test mutates a process-global env var (`KEBAB_EVAL_GOLDEN`) and
 /// expects to see its own write. Take this mutex around the body of
-/// every test that touches `KB_EVAL_GOLDEN` so two concurrent test
+/// every test that touches `KEBAB_EVAL_GOLDEN` so two concurrent test
 /// threads don't trip over each other's golden YAML.
 fn env_guard() -> std::sync::MutexGuard<'static, ()> {
     use std::sync::{Mutex, OnceLock};