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/hybrid.rs

@@ -157,17 +157,17 @@ fn hybrid_snapshot_run_1() {
         .join("hybrid")
         .join("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::create_dir_all(fixture.parent().unwrap()).unwrap();
         std::fs::write(&fixture, serde_json::to_string_pretty(&actual).unwrap()).unwrap();
         eprintln!("[snapshot] regenerated {}", fixture.display());
-        // Fail loudly so that accidentally setting KB_UPDATE_SNAPSHOTS
+        // Fail loudly so that accidentally setting KEBAB_UPDATE_SNAPSHOTS
         // in CI surfaces as a test failure rather than a silent
         // overwrite + green run. Same fail-loud-instead-of-silent-pass
         // philosophy as P3-2's `SNAPSHOT_HASH_BASELINE = 0` and P3-3's
         // placeholder fixture guards.
         panic!(
-            "[snapshot] regenerated {}, re-run without KB_UPDATE_SNAPSHOTS to verify pin",
+            "[snapshot] regenerated {}, re-run without KEBAB_UPDATE_SNAPSHOTS to verify pin",
             fixture.display()
         );
     }
@@ -176,7 +176,7 @@ fn hybrid_snapshot_run_1() {
         serde_json::from_str(&std::fs::read_to_string(&fixture).unwrap_or_else(|_| {
             panic!(
                 "missing snapshot fixture at {}; run with \
-                 KB_UPDATE_SNAPSHOTS=1 to create",
+                 KEBAB_UPDATE_SNAPSHOTS=1 to create",
                 fixture.display()
             )
         }))
@@ -189,14 +189,14 @@ fn hybrid_snapshot_run_1() {
         panic!(
             "snapshot fixture is a placeholder — regenerate on AVX hardware then commit. \
              Path: {}. To regenerate: \
-             `KB_UPDATE_SNAPSHOTS=1 cargo test -p kb-search -- --ignored hybrid_snapshot`.",
+             `KEBAB_UPDATE_SNAPSHOTS=1 cargo test -p kb-search -- --ignored hybrid_snapshot`.",
             fixture.display()
         );
     }
 
     assert_eq!(
         actual, expected,
-        "hybrid snapshot drift; rerun with KB_UPDATE_SNAPSHOTS=1 to regenerate"
+        "hybrid snapshot drift; rerun with KEBAB_UPDATE_SNAPSHOTS=1 to regenerate"
     );
 
     // Independent guard: fusion scores must be non-increasing across