Update go.mod require and main.go import to use the Gitea-hosted
module path go.moleculesai.app/plugin/gh-identity (migrated from
github.com/Molecule-AI/molecule-ai-plugin-gh-identity).
Follows the pattern of the org-template URL migrations (github.com ->
git.moleculesai.app) applied to Go module imports.
Fixes molecule-core#91.
Ref: molecule-internal#71.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>