feat: make MCP config file follow Claude directory override

When users set a custom Claude configuration directory, the MCP config
file (.claude.json) is now placed alongside the overridden directory
instead of the default ~/.claude.json location.

Changes:
- Add path derivation logic to generate MCP path from override directory
  (e.g., /custom/.claude → /custom/.claude.json)
- Implement automatic migration from legacy path on first access
- Add comprehensive unit tests covering 4 edge cases
- Update UI descriptions to clarify MCP file placement
- Fix documentation: correct MCP config path from ~/.claude/mcp.json
  to ~/.claude.json

Technical details:
- New function: derive_mcp_path_from_override() extracts directory name
  and creates sibling .json file
- Migration copies ~/.claude.json to new location if override is set
- All MCP operations (read/write/sync) now use the derived path via
  user_config_path() unified entry point

Breaking changes: None (backward compatible with default behavior)

src-tauri/src/claude_mcp.rs

@@ -4,7 +4,7 @@ use std::env;
 use std::fs;
 use std::path::{Path, PathBuf};
 
-use crate::config::atomic_write;
+use crate::config::{atomic_write, get_claude_mcp_path, get_default_claude_mcp_path};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -15,10 +15,49 @@ pub struct McpStatus {
 }
 
 fn user_config_path() -> PathBuf {
-    // 用户级 MCP 配置文件：~/.claude.json
-    dirs::home_dir()
-        .expect("无法获取用户主目录")
-        .join(".claude.json")
+    ensure_mcp_override_migrated();
+    get_claude_mcp_path()
+}
+
+fn ensure_mcp_override_migrated() {
+    if crate::settings::get_claude_override_dir().is_none() {
+        return;
+    }
+
+    let new_path = get_claude_mcp_path();
+    if new_path.exists() {
+        return;
+    }
+
+    let legacy_path = get_default_claude_mcp_path();
+    if !legacy_path.exists() {
+        return;
+    }
+
+    if let Some(parent) = new_path.parent() {
+        if let Err(err) = fs::create_dir_all(parent) {
+            log::warn!("创建 MCP 目录失败: {}", err);
+            return;
+        }
+    }
+
+    match fs::copy(&legacy_path, &new_path) {
+        Ok(_) => {
+            log::info!(
+                "已根据覆盖目录复制 MCP 配置: {} -> {}",
+                legacy_path.display(),
+                new_path.display()
+            );
+        }
+        Err(err) => {
+            log::warn!(
+                "复制 MCP 配置失败: {} -> {}: {}",
+                legacy_path.display(),
+                new_path.display(),
+                err
+            );
+        }
+    }
 }
 
 fn read_json_value(path: &Path) -> Result<Value, String> {