refactor(rename): kb crates → kebab — Cargo packages, folders, Rust modules

프로젝트 이름 `kb` → `kebab` rename 의 첫 단계.

- workspace `Cargo.toml`: members `crates/kb-*` → `crates/kebab-*`,
  repository URL `altair823/kb` → `altair823/kebab`.
- 18 crate 폴더 rename via `git mv` (history 보존).
- 각 crate `Cargo.toml`: `name = "kb-*"` → `"kebab-*"`, path deps
  `../kb-*` → `../kebab-*`.
- 모든 `.rs`: `kb_<id>` snake-case 모듈 path 18 개 (`kb_core`,
  `kb_config`, `kb_app`, `kb_cli`, `kb_eval`, `kb_search`, `kb_chunk`,
  `kb_normalize`, `kb_source_fs`, `kb_parse_md`, `kb_parse_types`,
  `kb_store_sqlite`, `kb_store_vector`, `kb_embed`, `kb_embed_local`,
  `kb_llm`, `kb_llm_local`, `kb_rag`) → `kebab_<id>` 일괄 sed (단어
  경계 \\b 사용해 영어 문장 안의 "kb" 약어 미오염).

CLI binary 이름 (`[[bin]] name = "kb"`), 환경변수 `KB_*`, XDG paths,
tracing target, 그리고 docs sweep 은 다음 commit 에서.

## 검증

- `cargo check --workspace` clean — 모든 crate 빌드 통과 후 commit.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-core/src/answer.rs

@@ -0,0 +1,66 @@
+//! Answer + RAG types (§3.8).
+
+use serde::{Deserialize, Serialize};
+use time::OffsetDateTime;
+
+use crate::citation::Citation;
+use crate::search::SearchMode;
+use crate::versions::PromptTemplateVersion;
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Answer {
+    pub answer: String,
+    pub citations: Vec<AnswerCitation>,
+    pub grounded: bool,
+    pub refusal_reason: Option<RefusalReason>,
+    pub model: ModelRef,
+    pub embedding: Option<ModelRef>,
+    pub prompt_template_version: PromptTemplateVersion,
+    pub retrieval: AnswerRetrievalSummary,
+    pub usage: TokenUsage,
+    #[serde(with = "time::serde::rfc3339")]
+    pub created_at: OffsetDateTime,
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct AnswerCitation {
+    pub marker: Option<String>,
+    pub citation: Citation,
+}
+
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum RefusalReason {
+    ScoreGate,
+    LlmSelfJudge,
+    NoIndex,
+    NoChunks,
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct ModelRef {
+    pub id: String,
+    pub provider: String,
+    pub dimensions: Option<usize>,
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct AnswerRetrievalSummary {
+    pub trace_id: TraceId,
+    pub mode: SearchMode,
+    pub k: usize,
+    pub score_gate: f32,
+    pub top_score: f32,
+    pub chunks_returned: u32,
+    pub chunks_used: u32,
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct TokenUsage {
+    pub prompt_tokens: u32,
+    pub completion_tokens: u32,
+    pub latency_ms: u32,
+}
+
+#[derive(Clone, Debug, Eq, Hash, PartialEq, Serialize, Deserialize)]
+pub struct TraceId(pub String);