fix: respect $VISUAL and $EDITOR env vars at runtime

Previously, env vars were only consulted during config generation
and baked into conf.yml. At runtime, the config file value was
always used, making it impossible to override the editor via
environment variables.

Now the precedence is: $VISUAL > $EDITOR > conf.yml > auto-detect.

Closes #589

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

cmd/cheat/config.go

@@ -3,7 +3,7 @@ package main
 // configs returns the default configuration template
 func configs() string {
 	return `---
-# The editor to use with 'cheat -e <sheet>'. Defaults to $EDITOR or $VISUAL.
+# The editor to use with 'cheat -e <sheet>'. Overridden by $VISUAL or $EDITOR.
 editor: EDITOR_PATH
 
 # Should 'cheat' always colorize output?