refactor: remove dead useMcpRegistration hook, add design file

useMcpRegistration is fully replaced by useMcpStatus which combines
detection + registration. Added design placeholder for MCP status bar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

design/mcp-autodetect-status-bar.pen

@@ -0,0 +1 @@
+{"children":[],"variables":{}}

src-tauri/src/mcp.rs

@@ -193,7 +193,11 @@ pub fn check_mcp_status() -> McpStatus {
     }
 
     // Verify the referenced index.js actually exists on disk
-    if let Some(index_js) = entry["args"].as_array().and_then(|a| a.first()).and_then(|v| v.as_str()) {
+    if let Some(index_js) = entry["args"]
+        .as_array()
+        .and_then(|a| a.first())
+        .and_then(|v| v.as_str())
+    {
         if !Path::new(index_js).exists() {
             return McpStatus::NotInstalled;
         }
@@ -381,7 +385,10 @@ mod tests {
         // On CI without Claude it might be NoClaudeCli. Either way it must not panic.
         let status = check_mcp_status();
         assert!(
-            matches!(status, McpStatus::Installed | McpStatus::NotInstalled | McpStatus::NoClaudeCli),
+            matches!(
+                status,
+                McpStatus::Installed | McpStatus::NotInstalled | McpStatus::NoClaudeCli
+            ),
             "unexpected status: {:?}",
             status
         );

src/hooks/useMcpRegistration.ts

@@ -1,33 +0,0 @@
-import { useEffect, useRef } from 'react'
-import { invoke } from '@tauri-apps/api/core'
-import { isTauri, mockInvoke } from '../mock-tauri'
-
-function tauriCall<T>(command: string, args: Record<string, unknown>): Promise<T> {
-  return isTauri() ? invoke<T>(command, args) : mockInvoke<T>(command, args)
-}
-
-/**
- * Registers Laputa as an MCP server in Claude Code and Cursor config files.
- * Fires once per vault path (skips duplicates).
- */
-export function useMcpRegistration(
-  vaultPath: string,
-  onToast: (msg: string) => void,
-) {
-  const registeredRef = useRef<string | null>(null)
-
-  useEffect(() => {
-    if (registeredRef.current === vaultPath) return
-    registeredRef.current = vaultPath
-
-    tauriCall<string>('register_mcp_tools', { vaultPath })
-      .then((status) => {
-        if (status === 'registered') {
-          onToast('Laputa registered as MCP tool for Claude Code')
-        }
-      })
-      .catch(() => {
-        // Silently ignore — not critical for app operation
-      })
-  }, [vaultPath]) // eslint-disable-line react-hooks/exhaustive-deps -- onToast is stable via ref
-}