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-source-fs/tests/snapshot_tree1.rs

@@ -9,8 +9,8 @@
 //! │   ├── alpha.md
 //! │   └── beta.md
 //! ├── ignored/
-//! │   └── skip.tmp           # excluded by .kbignore
-//! ├── .kbignore              # contains: *.tmp
+//! │   └── skip.tmp           # excluded by .kebabignore
+//! ├── .kebabignore              # contains: *.tmp
 //! └── .DS_Store              # implicitly excluded
 //! ```
 //!
@@ -52,7 +52,7 @@ fn cfg_for_fixture(root: &str) -> Config {
     let mut c = Config::defaults();
     c.workspace.root = root.to_string();
     // Clear default excludes (`.git/**`, `node_modules/**`, `.obsidian/**`)
-    // so the snapshot is purely a function of the fixture + .kbignore +
+    // so the snapshot is purely a function of the fixture + .kebabignore +
     // baked-in default-excludes.
     c.workspace.exclude.clear();
     c
@@ -101,18 +101,18 @@ fn scan_and_strip() -> Value {
 fn tree_1_snapshot_matches_baseline() {
     let actual = scan_and_strip();
 
-    // If KB_REGEN_SNAPSHOT is set, (re)write the baseline and exit
+    // If KEBAB_REGEN_SNAPSHOT is set, (re)write the baseline and exit
     // *before* attempting to read it. This is the only path that may
     // create the file from scratch.
-    if std::env::var_os("KB_REGEN_SNAPSHOT").is_some() {
+    if std::env::var_os("KEBAB_REGEN_SNAPSHOT").is_some() {
         let pretty = serde_json::to_string_pretty(&actual).unwrap() + "\n";
         std::fs::write(baseline_path(), pretty).expect("write baseline");
-        panic!("regenerated baseline; rerun without KB_REGEN_SNAPSHOT to verify");
+        panic!("regenerated baseline; rerun without KEBAB_REGEN_SNAPSHOT to verify");
     }
 
     let baseline_text = std::fs::read_to_string(baseline_path()).unwrap_or_else(|_| {
         panic!(
-            "missing baseline at {} — regenerate via `KB_REGEN_SNAPSHOT=1 cargo test \
+            "missing baseline at {} — regenerate via `KEBAB_REGEN_SNAPSHOT=1 cargo test \
              -p kb-source-fs --test snapshot_tree1 -- tree_1_snapshot_matches_baseline`",
             baseline_path().display()
         )