Remove conflicting gr LSP keymap

[stanfish06]

Motivation

• The buffer-local gr mapping shadows Neovim 0.11+ built-in gr* LSP mappings (grr/grn/gra/gri) and makes those built-ins unreachable when gr is set.

Description

• Removed the vim.keymap.set("n", "gr", vim.lsp.buf.references, opts) mapping from lua/config/plugin_config.lua to avoid shadowing the built-in gr* keys.
• Updated the comment in lua/config/options.lua to reflect that only gd remains set buffer-local via LspAttach.

Testing

• Ran git diff --check, which completed successfully.
• Attempted nvim --headless -u init.lua '+quit', but nvim is not installed in the environment so this check could not be run.
• Attempted to validate the Lua files with lua -e 'assert(loadfile("lua/config/plugin_config.lua")); assert(loadfile("lua/config/options.lua"))', but lua is not installed in the environment so this check could not be run.

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Codex Task