🏗️ refactor(fb-30): apply round 1 review nits

- error_wire.rs: extract `pub const ERROR_V1_ID = "error.v1"` + replace
  9 inline literals (parallel to schema.rs::SCHEMA_V1_ID pattern).
  Re-export via kebab-app::lib.rs.
- kebab-mcp/src/lib.rs: extract `KebabHandler::spawn_tool<I, F>` helper —
  search + ask arms reduce from ~17 lines each to a one-line dispatch.
  Future tool 추가 시 boilerplate 안 늘림.
- ask.rs: defensive `to_value(&answer)` — silent Null 위험 제거, 실패
  시 to_tool_error fallthrough.
- HOTFIXES: note AskOpts Default 미도입 limitation.
- ARCHITECTURE.md: directory tree 의 kebab-mcp 항목에 `schema` 추가
  (4 tool 모두 명시).

Round 1 review summary: #108 (comment)

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

docs/ARCHITECTURE.md

@@ -170,7 +170,7 @@ kebab/
 │   ├── kebab-parse-pdf/                               # lopdf per-page text extractor (P7-1)
 │   ├── kebab-app/                                     # facade (P0 시그니처 + P3-5/P6-4/P7-3 본체)
 │   ├── kebab-tui/                                     # Ratatui shell + Library 패널 (P9-1)
-│   ├── kebab-mcp/                                     # stdio MCP server — tools: search, ask, doctor (P9-FB-30)
+│   ├── kebab-mcp/                                     # stdio MCP server — tools: schema, doctor, search, ask (P9-FB-30)
 │   └── kebab-cli/                                     # binary (P0 → 핫픽스로 --config flag wiring 강화)
 ├── migrations/                                     # SQLite refinery V001/V002/V003
 └── fixtures/                                       # 테스트 fixture 트리