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-store-sqlite/src/store.rs

@@ -28,7 +28,7 @@ const ASSET_ID_HEX_LEN: usize = 32;
 /// Default file name under `config.storage.data_dir`. Kept private — the
 /// path layout is a §6.3 design decision, not part of the store's public
 /// surface.
-const SQLITE_FILE: &str = "kb.sqlite";
+const SQLITE_FILE: &str = "kebab.sqlite";
 
 /// Subdirectory under `data_dir` holding shard-prefixed asset bytes
 /// (`<aa>/<asset_id>`). Mirrors design §6.3.
@@ -74,7 +74,7 @@ impl SqliteStore {
         apply_pragmas(&conn)?;
 
         tracing::debug!(
-            target: "kb-store-sqlite",
+            target: "kebab-store-sqlite",
             data_dir = %data_dir.display(),
             db = %db_path.display(),
             "opened sqlite store"
@@ -94,7 +94,7 @@ impl SqliteStore {
         schema::runner()
             .run(&mut *conn)
             .map_err(|e| StoreError::Migration(e.to_string()))?;
-        tracing::debug!(target: "kb-store-sqlite", "migrations applied");
+        tracing::debug!(target: "kebab-store-sqlite", "migrations applied");
         Ok(())
     }