fix: remove cat from allowed shell commands and fix stale display tests

Remove cat from allowed_commands to close shell redirect bypass vector
(read_file provides safer alternative). Update display tests to match
render_user_message returning Text instead of Panel, and shell test to
use head instead of cat.

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

tests/unit/test_shell.py

@@ -90,6 +90,6 @@ class TestRunCommandTool:
         # Create a file in the workspace to verify cwd
         (ws / "marker.txt").write_text("found")
         tool = RunCommandTool(ws, cfg)
-        result = tool.run("tc-9", {"command": "cat marker.txt"})
+        result = tool.run("tc-9", {"command": "head marker.txt"})
         assert result.status == ToolResultStatus.SUCCESS
         assert "found" in result.output