GLSP-1682: Update references to the consolidated glsp-core repo

[tortmayr]

What it does

The client framework and the Node server framework moved into the eclipse-glsp/glsp-core monorepo; the former glsp-client and glsp-server-node repos will be archived.
This updates the references that would break or mislead after the archival. Docs and metadata only — no dependency or build changes.

• Point the docs at glsp-core: contribution guide, PR-target example URL, the landscape description in the README, and the Node server framework links of the node-json-* project templates
• Clarify that glsp-client/glsp-server in the README denote folders of an example, now that glsp-client collides with the name of the archived repo (no directories renamed)
• Fix the repository metadata of the workflow-glsp and workflow-server packages, which pointed at the soon-archived repos instead of glsp-examples
• Retarget the update-workflow agent skill at glsp-core so it keeps working after archival: it is now cloned once and provides both the client and the Node server dev example

Part of: eclipse-glsp/glsp#1682

How to test

Doc/metadata-only change; verified locally:

• Every new deep link resolves in the local glsp-core clone (packages/client, packages/server, examples/workflow-glsp, examples/workflow-server); default branch is main, so the tree/main/... links are correct
• No stale references left — grep for eclipse-glsp/glsp-client, eclipse-glsp/glsp-server-node, eclipse-glsp/glsp-playwright returns no hits (remaining glsp-client matches are local directory/package names)
• Both edited package.json files still parse as valid JSON
• The update-workflow skill's v<version> tag convention still holds — glsp-core carries the full tag history through v2.8.0

Follow-ups

• The build still uses yarn: root engines are yarn >=1.7 <2 / node >=14.18.0 while the README asks for Node >=20. The yarn → pnpm migration is a separate effort, intentionally out of scope here.
• @eclipse-glsp/* deps stay pinned at 2.8.0 until the first glsp-core release.
• This repo has no .github/PULL_REQUEST_TEMPLATE.md, unlike the other eclipse-glsp repos — worth adding separately.

Changelog

• This PR should be mentioned in the changelog
• This PR introduces a breaking change (if yes, provide more details below for the changelog and the migration guide)