feat(cards): add 'note' card type for readable markdown content

New card type renders body as styled markdown with no copy/fill/run
affordance. Glyph: ¶, color: --note.

Migration uses transaction to safely rebuild table constraint.
Checks both 'note' presence and modified_at column to catch
partial migration state.

internal/parse/grammar.go

@@ -27,6 +27,8 @@ var validCardTypes = map[string]string{
 	"checklist": "checklist",
 	"decision":  "decision",
 	"link":      "link",
+	"note":      "note",
+	"n":         "note",
 }
 
 func Parse(input string) (*Result, error) {