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-rag/src/pipeline.rs

@@ -135,7 +135,7 @@ impl RagPipeline {
         let top_score = hits.first().map(|h| h.retrieval.fusion_score).unwrap_or(0.0);
 
         tracing::debug!(
-            target: "kb-rag",
+            target: "kebab-rag",
             chunks_returned,
             top_score,
             mode = ?opts.mode,
@@ -161,7 +161,7 @@ impl RagPipeline {
         // collapse to the more accurate `NoChunks` refusal here.
         if packed_entries.is_empty() {
             tracing::warn!(
-                target: "kb-rag",
+                target: "kebab-rag",
                 chunks_returned = hits.len(),
                 "kb-rag: all retrieved chunks were unfetchable from the store; \
                  falling back to NoChunks refusal"
@@ -324,7 +324,7 @@ impl RagPipeline {
         // Drop the moved `finish_reason` early into a tracing breadcrumb; the
         // wire schema does not surface it (per design §3.8).
         tracing::debug!(
-            target: "kb-rag",
+            target: "kebab-rag",
             grounded = answer.grounded,
             refusal = ?answer.refusal_reason,
             refusal_phrase_detected = matched_refusal_phrase,
@@ -354,7 +354,7 @@ impl RagPipeline {
             self.docs.put_answer(&answer, query, packed_chunks_json.as_deref())
         {
             tracing::warn!(
-                target: "kb-rag",
+                target: "kebab-rag",
                 error = %e,
                 "kb-rag: put_answer failed; in-memory Answer still returned"
             );
@@ -386,7 +386,7 @@ impl RagPipeline {
                 Some(c) => c.text,
                 None => {
                     tracing::warn!(
-                        target: "kb-rag",
+                        target: "kebab-rag",
                         chunk_id = %hit.chunk_id.0,
                         "kb-rag: chunk not found in store; skipping"
                     );
@@ -454,7 +454,7 @@ impl RagPipeline {
             created_at: OffsetDateTime::now_utc(),
         };
         if let Err(e) = self.docs.put_answer(&answer, query, None) {
-            tracing::warn!(target: "kb-rag", error = %e, "kb-rag: put_answer (NoChunks) failed");
+            tracing::warn!(target: "kebab-rag", error = %e, "kb-rag: put_answer (NoChunks) failed");
         }
         Ok(answer)
     }
@@ -529,7 +529,7 @@ impl RagPipeline {
             created_at: OffsetDateTime::now_utc(),
         };
         if let Err(e) = self.docs.put_answer(&answer, query, None) {
-            tracing::warn!(target: "kb-rag", error = %e, "kb-rag: put_answer (ScoreGate) failed");
+            tracing::warn!(target: "kebab-rag", error = %e, "kb-rag: put_answer (ScoreGate) failed");
         }
         Ok(answer)
     }