Let rules carry a note

A glob like *4412* says nothing about why it exists or who it catches, and
six months later neither does memory. Rules get an optional note: free text
that never takes part in matching, written as a TOML key rather than a
comment so it survives a round trip and can be shown back.

The rule builder grows a fourth field for it and the rules screen a last
column. Four fields spaced out are taller than a short window has room for,
so the form now drops its blank lines and then the hints on unfocused fields
before anything would run off the bottom.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -69,6 +69,11 @@ could match** — `Engine.Usage` counts by `MatchIndex`, so a rule shadowed by a
 earlier one correctly reports zero. That is what makes the rules screen able to
 find dead rules at all.
 
+**A rule's `note` is documentation that round-trips.** It is a TOML key rather
+than a `#` comment so `LoadRules` can return it, the builder can write it and
+the rules screen can show it. It never takes part in matching — `rules.Engine`
+does not look at it — and it must stay that way.
+
 **`config.DeleteRules` edits rules.toml textually, never by re-serialising the
 parsed rules**, because comments and formatting are not recoverable from
 `[]Rule`. A rule owns the comment lines directly above it; a comment separated