Consume the published inkwell module instead of a sibling checkout

inkwell is now released as engines/inkwell v0.1.0, so the `replace
=> ../inkwell` directive can go. That directive is why this pipeline
was failing: it requires a sibling working copy that only exists on a
developer's machine, never in a CI checkout.

Module paths follow the org move as well — this repo is demos/, inkwell
is engines/ — since Gitea's redirect does not apply to Go module paths.

GOPRIVATE is exported from the Makefile: our own modules are not on
proxy.golang.org and not in the public checksum database.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

lib/character.cat.go

@@ -3,7 +3,7 @@ package lib
 import (
 	"image/color"
 
-	p "git.teletypegames.org/games/inkwell"
+	p "git.teletypegames.org/engines/inkwell"
 )
 
 var Cat = p.Character{