Security/Bug: Go server leaks auth_token to ps and leaves zombie processes

## Prerequsities

- [x] I'm on the latest version of the plugin
- [x] I've installed the required dependencies
- [ ] I've run `:h gitlab.nvim.troubleshooting` and followed the steps there

## Setup Configuration and Environment

NVIM v0.12.4
Build type: Release
LuaJIT 2.1.1784580905
Vim versions: 8.1, 8.2, 9.0, 9.1, 9.2

system vimrc file: "$VIM/sysinit.vim"
fall-back for $VIM: "/usr/local/Cellar/neovim/0.12.4/share/nvim"

## Bug Description

Currently, gitlab.nvim passes its configuration JSON (which contains the auth_token) directly
to the Go server binary via a command-line argument when calling vim.system().

Because command-line arguments are globally visible on UNIX systems via ps or pgrep, any user
on the machine can easily read the GitLab API token just by listing running processes.

Additionally, if Neovim is closed forcefully or quickly (e.g. via scripts or aliases), the Go
server processes aren't killed and become orphaned zombies running in the background. Because
they pile up, it becomes very easy to accidentally spot the leaked token in pgrep -fl nvim outputs.

## Reproduction Steps

1. Check out the feature branch
2. Open Neovim
3. ...

## Screenshots
```
➜ pgrep -fl nvim
2080 /Users/xxxxxxx/.local/share/nvim/gitlab.nvim/bin/server {"gitlab_url":"https://xxxxxxxxxxxxxxx.com","connection_settings":{"insecure":false,"proxy":"","remote":"origin"},"debug":{"response":false,"request":false,"gitlab_request":false,"gitlab_response":false},"port":54232,"chosen_mr_iid":0,"log_path":"/Users/xxxxxxx/.cache/nvim/gitlab.nvim.log","auth_token":"xxxxxxxxxxxxxxxxx"}
21519 nvim . .
21544 nvim --embed . .
66859 /Users/xxxxxxx/.local/share/nvim/gitlab.nvim/bin/server {"chosen_mr_iid":4143,"log_path":"/Users/xxxxxxx/.cache/nvim/gitlab.nvim.log","auth_token":"xxxxxxxxxxxxxxxxx","gitlab_url":"https://xxxxxxxxxxxxxxx.com","debug":{"request":false,"gitlab_request":false,"gitlab_response":false,"response":false},"connection_settings":{"proxy":"","remote":"origin","insecure":false},"port":51887}
83060 /Users/xxxxxxx/.local/share/nvim/gitlab.nvim/bin/server {"debug":{"gitlab_request":false,"gitlab_response":false,"response":false,"request":false},"connection_settings":{"remote":"origin","proxy":"","insecure":false},"port":0,"chosen_mr_iid":0,"log_path":"/Users/xxxxxxx/.cache/nvim/gitlab.nvim.log","auth_token":"xxxxxxxxxxxxxxxxx","gitlab_url":"https://xxxxxxxxxxxxxxx.com"}
84503 /Users/xxxxxxx/.local/share/nvim/gitlab.nvim/bin/server {"debug":{"gitlab_request":false,"gitlab_response":false,"response":false,"request":false},"connection_settings":{"remote":"origin","proxy":"","insecure":false},"port":0,"chosen_mr_iid":0,"log_path":"/Users/xxxxxxx/.cache/nvim/gitlab.nvim.log","auth_token":"xxxxxxxxxxxxxxxxx","gitlab_url":"https://xxxxxxxxxxxxxxx.com"}
91585 /Users/xxxxxxx/.local/share/nvim/gitlab.nvim/bin/server {"log_path":"/Users/xxxxxxx/.cache/nvim/gitlab.nvim.log","port":0,"gitlab_url":"https://xxxxxxxxxxxxxxx.com","debug":{"request":false,"gitlab_request":false,"gitlab_response":false,"response":false},"connection_settings":{"insecure":false,"proxy":"","remote":"origin"},"auth_token":"xxxxxxxxxxxxxxxxx","chosen_mr_iid":0}
95389 /Users/xxxxxxx/.local/share/nvim/gitlab.nvim/bin/server {"chosen_mr_iid":0,"log_path":"/Users/xxxxxxx/.cache/nvim/gitlab.nvim.log","auth_token":"xxxxxxxxxxxxxxxxx","debug":{"gitlab_response":false,"response":false,"request":false,"gitlab_request":false},"gitlab_url":"https://xxxxxxxxxxxxxxx.com","connection_settings":{"proxy":"","insecure":false,"remote":"origin"},"port":0}
97335 /Users/xxxxxxx/.local/share/nvim/gitlab.nvim/bin/server {"debug":{"response":false,"request":false,"gitlab_request":false,"gitlab_response":false},"chosen_mr_iid":0,"log_path":"/Users/xxxxxxx/.cache/nvim/gitlab.nvim.log","auth_token":"xxxxxxxxxxxxxxxxx","gitlab_url":"https://xxxxxxxxxxxxxxx.com","connection_settings":{"insecure":false,"remote":"origin","proxy":""},"port":0}
```

## Suggested Fix

The configuration JSON (or at least the auth_token) should be passed to the Go server via stdin
or as an environment variable, rather than as a command-line argument. Additionally, it would
be great if the plugin ensured the server process is killed properly upon VimLeavePre.

Start with :h gitlab.nvim.troubleshooting, then locate the vim.system() server launch, Go configuration handling, and VimLeavePre lifecycle handling. The report names no source files or tests and leaves reproduction incomplete. Done means authentication still works without exposing the token in process arguments, and server processes do not remain after Neovim exits, including the reported rapid or forced exits.

Written by the indexing model from the issue text.