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>

go.mod

@@ -1,9 +1,9 @@
-module git.teletypegames.org/games/inkwell-demo
+module git.teletypegames.org/demos/inkwell-demo
 
 go 1.26.3
 
 require (
-	git.teletypegames.org/games/inkwell v0.0.0-00010101000000-000000000000
+	git.teletypegames.org/engines/inkwell v0.1.0
 	github.com/hajimehoshi/ebiten/v2 v2.9.9
 )
 
@@ -19,5 +19,3 @@ require (
 	golang.org/x/sync v0.17.0 // indirect
 	golang.org/x/sys v0.36.0 // indirect
 )
-
-replace git.teletypegames.org/games/inkwell => ../inkwell