Remove CLAUDE.md symlinks in favor of AGENTS.md (#98910)
### What?
Removes the three `CLAUDE.md` symlinks that point at `AGENTS.md` (repo
root, `.github/`, and `turbopack/`), drops the "CLAUDE.md is a symlink"
note from `AGENTS.md`, and removes the now-unused `CLAUDE.md` entry from
`.alexignore`.
### Why?
Claude Code now reads `AGENTS.md` directly, so the symlinks are
redundant. Keeping a single instruction file avoids the two drifting
apart.
### Notes
- The `CLAUDE.md` fixtures under `evals/next-upgrade/evals/*` are
intentionally untouched. They simulate user projects for the upgrade
evals rather than repo instructions.
- No changes to `create-next-app` or `@next/codemod`, which still
generate `CLAUDE.md` for user projects.
