fix(install): bundle migrations in release tarball and install script (#249)

The install script only copied the binary, so `goclaw migrate up` and
`goclaw onboard` failed when run from the installed location because the
migrations directory was missing.

- Include migrations/ in the release tarball
- Install migrations to /usr/local/share/goclaw/migrations
- Guide users to export GOCLAW_MIGRATIONS_DIR before onboard
- Align next-steps output with onboard's own instructions

.github/workflows/release.yaml

@@ -57,7 +57,7 @@ jobs:
           CGO_ENABLED=0 go build \
             -ldflags="-s -w -X github.com/nextlevelbuilder/goclaw/cmd.Version=${VERSION}" \
             -o goclaw .
-          tar -czf "goclaw-${{ needs.release.outputs.version }}-${{ matrix.goos }}-${{ matrix.goarch }}.tar.gz" goclaw
+          tar -czf "goclaw-${{ needs.release.outputs.version }}-${{ matrix.goos }}-${{ matrix.goarch }}.tar.gz" goclaw migrations/
 
       - name: Upload to release
         env: