🏗️ 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>

crates/kebab-mcp/src/tools/ask.rs

@@ -50,7 +50,10 @@ pub fn handle(state: &KebabAppState, input: AskInput) -> CallToolResult {
         Ok(answer) => {
             // `Answer` does not carry `schema_version`; tag inline (idempotent
             // via entry().or_insert_with in case a future version adds it).
-            let mut v = serde_json::to_value(&answer).unwrap_or_default();
+            let mut v = match serde_json::to_value(&answer) {
+                Ok(v) => v,
+                Err(e) => return to_tool_error(&anyhow::anyhow!("answer serialize failed: {e}")),
+            };
             if let serde_json::Value::Object(ref mut map) = v {
                 map.entry("schema_version".to_string())
                     .or_insert_with(|| serde_json::Value::String("answer.v1".to_string()));