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.sum

@@ -1,3 +1,5 @@
+git.teletypegames.org/engines/inkwell v0.1.0 h1:NJgT924aR3e0uLVRiTEhzLrXJY+Vnj+FArkKqMJJlVI=
+git.teletypegames.org/engines/inkwell v0.1.0/go.mod h1:/v6QtismTE8+e7kobbVR5B/CpTSrd4j2DYwJ8DvINhs=
 github.com/ebitengine/gomobile v0.0.0-20250923094054-ea854a63cce1 h1:+kz5iTT3L7uU+VhlMfTb8hHcxLO3TlaELlX8wa4XjA0=
 github.com/ebitengine/gomobile v0.0.0-20250923094054-ea854a63cce1/go.mod h1:lKJoeixeJwnFmYsBny4vvCJGVFc3aYDalhuDsfZzWHI=
 github.com/ebitengine/hideconsole v1.0.0 h1:5J4U0kXF+pv/DhiXt5/lTz0eO5ogJ1iXb8Yj1yReDqE=