feat: E edits the config file

E opens your config like e opens a file: in the nvim lazysecrets runs
in, else in $VISUAL or $EDITOR, else in the system's default app. A
missing config is made first, with a link to its docs. Closing the
editor says to restart lazysecrets to use the changes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/config.md

@@ -2,7 +2,9 @@
 
 Optional, at `~/.config/lazysecrets/config.yaml` (or
 `$XDG_CONFIG_HOME/lazysecrets/config.yaml`, or `--config`). Flags override
-it. Unknown keys are an error, so typos don't go unnoticed.
+it. Unknown keys are an error, so typos don't go unnoticed. `E` in
+lazysecrets opens it in your editor (making it if it isn't there yet);
+changes apply when lazysecrets starts again.
 
 ```yaml
 icons: true            # Nerd Font icons (v3+); also --icons=false or LAZYSECRETS_ICONS=0