ci: support manual npm publish

[fbosch]

Summary

• Add an optional publish_tag input to the Release Please workflow.
• Allow the publish job to run for an existing release tag when Release Please does not create a new release.

Validation

• Workflow YAML is ignored by the repo Biome config, so no local formatter check applies.

Backfill

After merge, run Release Please manually with publish_tag=opencode-toon-plugin-v0.1.2 to publish the already-created GitHub release to npm.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/opencode-toon-plugin@13

commit: 50e8de2