🏗️ 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-app/src/error_wire.rs

@@ -11,6 +11,10 @@ use serde_json::{Value, json};
 
 use crate::error_signal::{ConfigInvalid, LlmError, NotIndexed};
 
+/// Wire schema id for [`ErrorV1`]. Single source of truth — kebab-cli
+/// + kebab-mcp use this via `kebab_app::ERROR_V1_ID`.
+pub const ERROR_V1_ID: &str = "error.v1";
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ErrorV1 {
     pub schema_version: String,
@@ -23,7 +27,7 @@ pub struct ErrorV1 {
 pub fn classify(err: &anyhow::Error, verbose: bool) -> ErrorV1 {
     if let Some(s) = err.downcast_ref::<ConfigInvalid>() {
         return ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "config_invalid".to_string(),
             message: s.to_string(),
             details: json!({
@@ -35,7 +39,7 @@ pub fn classify(err: &anyhow::Error, verbose: bool) -> ErrorV1 {
     }
     if let Some(s) = err.downcast_ref::<NotIndexed>() {
         return ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "not_indexed".to_string(),
             message: s.to_string(),
             details: json!({
@@ -50,7 +54,7 @@ pub fn classify(err: &anyhow::Error, verbose: bool) -> ErrorV1 {
     }
     if let Some(io) = err.downcast_ref::<std::io::Error>() {
         return ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "io_error".to_string(),
             message: io.to_string(),
             details: json!({"kind": format!("{:?}", io.kind())}),
@@ -63,7 +67,7 @@ pub fn classify(err: &anyhow::Error, verbose: bool) -> ErrorV1 {
         details = json!({"chain": chain});
     }
     ErrorV1 {
-        schema_version: "error.v1".to_string(),
+        schema_version: ERROR_V1_ID.to_string(),
         code: "generic".to_string(),
         message: err.to_string(),
         details,
@@ -74,7 +78,7 @@ pub fn classify(err: &anyhow::Error, verbose: bool) -> ErrorV1 {
 fn classify_llm(s: &LlmError) -> ErrorV1 {
     match s {
         LlmError::Unreachable { endpoint, source } => ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "model_unreachable".to_string(),
             message: format!("ollama unreachable at {endpoint}"),
             details: json!({
@@ -84,28 +88,28 @@ fn classify_llm(s: &LlmError) -> ErrorV1 {
             hint: Some(format!("ensure `ollama serve` is reachable at {endpoint}")),
         },
         LlmError::ModelNotPulled(model) => ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "model_not_pulled".to_string(),
             message: format!("ollama model `{model}` is not pulled"),
             details: json!({"model": model}),
             hint: Some(format!("run `ollama pull {model}`")),
         },
         LlmError::Timeout(e) => ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "timeout".to_string(),
             message: format!("ollama timeout: {e}"),
             details: json!({"source": e.to_string()}),
             hint: Some("increase timeout or check Ollama load".to_string()),
         },
         LlmError::Stream(body) => ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "generic".to_string(),
             message: format!("ollama HTTP error: {body}"),
             details: json!({"body": body}),
             hint: None,
         },
         LlmError::Malformed(line) => ErrorV1 {
-            schema_version: "error.v1".to_string(),
+            schema_version: ERROR_V1_ID.to_string(),
             code: "generic".to_string(),
             message: format!("malformed response line: {line}"),
             details: json!({"line": line}),

crates/kebab-app/src/lib.rs

@@ -66,7 +66,7 @@ pub mod schema;
 pub use app::App;
 pub use ingest_progress::{AggregateCounts, IngestEvent, render_skipped_breakdown};
 pub use reset::{ResetReport, ResetScope};
-pub use error_wire::{ErrorV1, classify};
+pub use error_wire::{ERROR_V1_ID, ErrorV1, classify};
 pub use schema::{Capabilities, Models, SCHEMA_V1_ID, SchemaV1, Stats, WireBlock, schema_with_config};
 
 /// p9-fb-25: sentinel for files without an extension in