OpenCode 1.15.x's TUI plugin loader runs npm install per spec, which fails
on subpath specs like @riskihajar/opencode-omniroute-auth/tui because npm
treats the slash as a local path (NpmInstallFailedError).

Installer now writes the absolute path to dist/tui.js into tui.json, and
migrates any legacy subpath entry on the next install run. Stale absolute
paths from prior install locations are also cleaned up.