GLSP-1682: Align repository with glsp-core and modernize the client toolchain

[tortmayr]

What it does

Aligns this repository with glsp-core: the agent setup the other repositories
already share, and the client toolchain that @eclipse-glsp/dev now provides.
@eclipse-glsp/eslint-config stopped at 2.9.0-next.1 and is no longer part of the
meta package, so the ESLint/Prettier setup would otherwise freeze where it is.

Agent setup

• Replace the generated "Repository Guidelines" in AGENTS.md with the minimal
  bullet form the other repos use: only what is not derivable from the code, such as
  the client/server split and the copy:client step the example plugin depends on.
  CLAUDE.md only imports it, and client/CLAUDE.md is dropped — its @AGENTS.md
  import pointed at a file that never existed
• Extend the /fix skill to both halves: the client workspace and the Maven build
  with the checkstyle summary, mirroring the CI split that collects violations
  instead of aborting at validate
• Mirror both skills under .agents for agents that do not read .claude, and stop
  the changelog skill from being invoked implicitly
• Point README and CONTRIBUTING at glsp-core instead of glsp-client

Client toolchain

• Node 24 and pnpm 12, with the workflows moved to node-version: 24 and
  pnpm/action-setup v6.1.0 (first version that can install pnpm 12)
• oxlint and oxfmt replace ESLint and Prettier: two new configs; .prettierrc,
  eslint.config.mjs, tsconfig.eslint.json and the per-package lint scripts are
  gone. oxlint type-checks the same program it lints, so pnpm lint needs no prior
  build, and the CI lint job emits inline annotations plus a summary table through
  lint-report.mjs instead of routing ESLint JSON through a third-party converter
• TypeScript 7.0, and the GLSP dependencies pinned to the current next versions as
  in the theia and vscode repositories
• Editor configuration points at the oxc extension and the TypeScript native preview,
  since the TypeScript 7 package ships no tsserver

Also:

• Exclude .claude/** and .agents/** from the CI triggers and tidy the agent
  entries in .gitignore

Part of: eclipse-glsp/glsp#1682

How to test

cd client
pnpm install
pnpm check:all   # build + oxlint (incl. type check) + oxfmt --check + headers

• Verified locally: pnpm check:all passes end to end (build, lint, format check, and the header check across the TS and Java sources)
• Verified locally: oxlint reports TypeScript diagnostics and missing copyright headers (checked with a deliberately broken probe file)
• Verified locally: pnpm copy:client still copies the bundle into the workflow editor plugin
• The Maven/Tycho server build was not run locally (unchanged by this PR); CI (Server) covers it

Follow-ups

• The next pins are snapshots (2.9.0-next.3 / 2.9.0-next.4); pnpm upgrade:next refreshes them before the next release.
• glsp-core additionally carries a "Contributing code / human-in-the-loop" section in
  CONTRIBUTING.md that none of the integration repositories 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)