GLSP-1682: Align repository with the glsp-core mono repo

[tortmayr]

What it does

The client, protocol, Node server and Playwright framework now live in
eclipse-glsp/glsp-core, so the E2E setup and the repository references have to
follow.

• Rewire the E2E workflow: build the Workflow fatjar here, start it as a WebSocket
  server and run the standalone Playwright suites of glsp-core against it with
  GLSP_SERVER_TYPE=java, which makes the suites assert the Java server labels and
  skip the web-worker project. The glsp-client/glsp-playwright checkouts and the
  glsp repo CLI they provided are gone, so the server is started and awaited
  explicitly
• Adopt the agent setup of glsp-core: AGENTS.md is the single source and
  CLAUDE.md only imports it, /verify becomes /fix (headers, build, checkstyle,
  tests) and both skills are mirrored under .agents for agents that do not read
  .claude
• Point README and CONTRIBUTING at glsp-core instead of glsp-client and list
  the standalone client the README already promised

Also:

• Exclude .claude/** and .agents/** from the CI and E2E triggers

Part of: eclipse-glsp/glsp#1682

How to test

Run the E2E workflow from the Actions tab (workflow_dispatch), or reproduce it
locally:

mvn clean package -Pm2 -Pfatjar -B -DskipTests -Dcheckstyle.skip
java -jar examples/org.eclipse.glsp.example.workflow/target/*-glsp.jar --websocket --port 8081 &

# in a glsp-core checkout
pnpm install --frozen-lockfile && pnpm build
GLSP_SERVER_TYPE=java pnpm e2e test:standalone

• Verified locally: 73 standalone Playwright tests pass against the Java server
• Verified locally: the /fix steps run green (scoped header check, mvn clean verify -Pm2 -Pfatjar, checkstyle summary reports no violations)

Follow-ups

• The E2E job tracks the default branch of glsp-core; if that turns out to be too
  volatile, the checkout can be pinned to a tag.
• glsp-core additionally carries a "Contributing code / human-in-the-loop" section
  in CONTRIBUTING.md that neither this repo nor the Theia/VS Code integrations have
  yet — worth aligning across all repos in a separate PR.

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)