fix(mcp): only validate projectPath when it exists; add handler tests

#230's validateProjectPath ran on the raw path incl. a statSync existence
check, which rejected the nested/not-yet-created sub-paths that issue #238
expects to resolve UP to the default project. Guard with existsSync so a
direct sensitive dir (e.g. /etc, C:\Windows) is still refused while
sub-paths walk up. Adds MCP-handler rejection tests (POSIX + Windows-gated).

Author: colbymchenry

__tests__/security.test.ts

@@ -277,6 +277,34 @@ describe('MCP Input Validation', () => {
     const result = await handler.execute('codegraph_search', { query: 'example', limit: -5 });
     expect(result.isError).toBeFalsy();
   });
+
+  // #230: getCodeGraph must reject a sensitive system directory passed as
+  // projectPath before opening it. The error surfaces through execute()'s
+  // catch as an isError result. /etc is sensitive on POSIX; C:\Windows on
+  // Windows (path.resolve is platform-specific, so each case is gated).
+  it.runIf(process.platform !== 'win32')(
+    'rejects a sensitive POSIX projectPath (/etc) via the MCP handler',
+    async () => {
+      const result = await handler.execute('codegraph_search', {
+        query: 'example',
+        projectPath: '/etc',
+      });
+      expect(result.isError).toBe(true);
+      expect(result.content[0].text).toMatch(/sensitive system directory/i);
+    }
+  );
+
+  it.runIf(process.platform === 'win32')(
+    'rejects a sensitive Windows projectPath (C:\\Windows) via the MCP handler',
+    async () => {
+      const result = await handler.execute('codegraph_search', {
+        query: 'example',
+        projectPath: 'C:\\Windows',
+      });
+      expect(result.isError).toBe(true);
+      expect(result.content[0].text).toMatch(/sensitive system directory/i);
+    }
+  );
 });
 
 describe('Atomic Writes', () => {

src/mcp/tools.ts

@@ -579,10 +579,16 @@ export class ToolHandler {
       return this.projectCache.get(projectPath)!;
     }
 
-    // Validate the path is safe before opening
-    const pathError = validateProjectPath(projectPath);
-    if (pathError) {
-      throw new Error(pathError);
+    // Reject sensitive system directories before opening. Only validate a
+    // path that actually exists — a nested or not-yet-created sub-path of a
+    // real project must still be allowed to resolve UP to its .codegraph/
+    // root below (issue #238), so we don't run the existence-checking
+    // validator on paths that are meant to walk up.
+    if (existsSync(projectPath)) {
+      const pathError = validateProjectPath(projectPath);
+      if (pathError) {
+        throw new Error(pathError);
+      }
     }
 
     // Walk up parent directories to find nearest .codegraph/