fix(tui): remove unused shutil import; implement width-based auto_diff

Remove the unused `shutil` import from models.py (fixes lint F401).

Implement `auto_diff()` with actual terminal-width-based dispatch as
spec §30139 requires: terminals >= 120 columns get split (side-by-side)
format, narrower terminals get unified diff. Previously the method
called `difflib.context_diff()` with a docstring falsely claiming
width-based selection.

ISSUES CLOSED: #1480

src/cleveragents/tui/permissions/models.py

@@ -6,7 +6,6 @@ tool permission requests with diff views and record user decisions.
 
 from __future__ import annotations
 
-import shutil
 from enum import StrEnum
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -140,23 +139,21 @@ class PermissionRequest(BaseModel):
         return left, right
 
     def auto_diff(self, *, context_lines: int = 3) -> str:
-        """Generate a context diff showing only changed lines with context.
+        """Select unified or split diff based on available terminal width.
 
-        In AUTO mode, the display mode is selected based on available
-        terminal width: unified for narrow terminals, split for wide.
+        Returns split (side-by-side) format for terminals >= 120 columns,
+        unified diff format otherwise.
         """
-        import difflib
+        import shutil
 
-        before_lines = (self.before_content or "").splitlines(keepends=True)
-        after_lines = (self.after_content or "").splitlines(keepends=True)
-        diff_lines = difflib.context_diff(
-            before_lines,
-            after_lines,
-            fromfile=f"a/{self.path}",
-            tofile=f"b/{self.path}",
-            n=context_lines,
-        )
-        return "".join(diff_lines)
+        terminal_width = shutil.get_terminal_size().columns
+        if terminal_width >= 120:
+            left, right = self.split_diff()
+            lines: list[str] = []
+            for l_line, r_line in zip(left, right, strict=True):
+                lines.append(f"{l_line:<40} | {r_line}")
+            return "\n".join(lines)
+        return self.unified_diff(context_lines=context_lines)
 
     def render_diff(
         self,