nix: remove stale packages/shared filter from node_modules.nix

[jerome-benoit]

Description

nix/node_modules.nix still has --filter './packages/shared' but that package was renamed to packages/core in #24309. The filter is inert — bun silently ignores it and the nix build passes regardless (core's deps are hoisted via packages/opencode). Dead code to clean up.

OpenCode version

dev branch (post #24309)

Steps to reproduce

N/A — not a build failure, just dead config.

Operating System

Any

Terminal

N/A