feat: auto permission mode, edit diffs, truncated tool output

- Default permission mode changed to 'auto' (read-only auto-allows,
  writes prompt)
- fs.edit now shows diff-style output: line numbers, context ±3 lines,
  + for added (green), - for removed (red)
- Tool output truncated to 10 lines in TUI with "+N lines (Ctrl+O
  to expand)" indicator
- Mistral SDK bumped to v1.3.0

internal/tui/theme.go

@@ -85,6 +85,12 @@ var (
 
 	sCursor = lipgloss.NewStyle().
 		Foreground(cPurple)
+
+	sDiffAdd = lipgloss.NewStyle().
+		Foreground(cGreen)
+
+	sDiffRemove = lipgloss.NewStyle().
+			Foreground(cRed)
 )
 
 // Status bar