docs(plans): say what GoField is in the policy example

A fact carries two names for one field, Field and GoField, and the
example gave no reason for the second. Field is the schema's name, which
rules match on and which names the column. GoField is the Go struct
field that holds the value, which the generator reads from and reuses as
the name of the encrypted type's field.

Refs #1046

Claude-Session: https://claude.ai/code/session_014jiJy4WhYH2LXojK2jSoxi

Author: auxesis

docs/plans/2026-10-04-plan-builder/policy/policy.go

@@ -16,6 +16,12 @@ func categories(values ...string) []plan.Annotation {
 
 // Source states the facts a schema reader would produce. A protobuf source
 // reads the same facts from descriptors and their custom options.
+//
+// A fact has two names because the schema and the Go struct spell a field
+// differently. Field is the schema's name: rules match on it, and it names
+// the column unless plan.Column sets another. GoField is the field of
+// individuals.Individual that holds the value, and the generator gives the
+// field of EncryptedIndividual the same name. With no GoField, Field is both.
 var Source = plan.SourceFunc(func(msg any) ([]plan.Fact, error) {
 	return []plan.Fact{
 		{Field: "id", GoField: "ID"},