feat: distinguish new notes (green dot) from modified notes (orange dot)

New notes created in-session show a green dot; existing notes with
uncommitted git changes show an orange dot. Saving a new note clears
the green dot; git commit clears the orange dot.

Introduces NoteStatus type ('new' | 'modified' | 'clean'), extracts
useNewNoteTracker hook and resolveNoteStatus pure function, and adds
onNoteSaved callback to useEditorSave for clearing new-note tracking.

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

src/components/Editor.test.tsx

@@ -206,20 +206,33 @@ describe('Editor', () => {
         {...defaultProps}
         tabs={[mockTab]}
         activeTabPath={mockEntry.path}
-        isModified={() => true}
+        getNoteStatus={() => 'modified'}
       />
     )
     // Modified indicator shows "M" in the breadcrumb
     expect(screen.getByText('M')).toBeInTheDocument()
   })
 
+  it('shows new indicator when file is new', () => {
+    render(
+      <Editor
+        {...defaultProps}
+        tabs={[mockTab]}
+        activeTabPath={mockEntry.path}
+        getNoteStatus={() => 'new'}
+      />
+    )
+    // New indicator shows "N" in the breadcrumb
+    expect(screen.getByText('N')).toBeInTheDocument()
+  })
+
   it('renders diff toggle button when file is modified', () => {
     render(
       <Editor
         {...defaultProps}
         tabs={[mockTab]}
         activeTabPath={mockEntry.path}
-        isModified={() => true}
+        getNoteStatus={() => 'modified'}
         onLoadDiff={async () => '+ added line'}
       />
     )