ci: release-please manages one package; the workflow tags the sub-modules

[giraffesyo]

The first real release PR (#23, 0.1.2) touches only the root module: with three packages and linked-versions, release-please only proposes a release for a package that has commits under its own path, and #22 changed nothing under hopperotel/ or hopperui/. The plugin keeps versions in sync among the packages it releases; it does not add the others. So the sub-modules would have stayed at v0.1.1 and the one-version promise in docs/releasing.md would have been broken on the very first automated release.

This makes release-please manage one package, the core module, with the sub-modules' go.mod requirement rewritten through extra-files (the // x-release-please-version lines), and has the release workflow tag hopperotel/vX.Y.Z and hopperui/vX.Y.Z on the release commit before goreleaser runs. One changelog, one GitHub release, three tags at one version, from any change anywhere in the repository. The manifest drops the sub-module entries.

Merge this before #23. release-please re-evaluates on the next push to canary and rewrites #23 under the new configuration (it should then also carry the two go.mod bumps). docs/releasing.md is updated; actionlint passes.