feat(config): add config path command

Description

The global configuration location varies by platform, and by whether a legacy ~/.config/glab-cli directory exists:

1. $GLAB_CONFIG_DIR when set.
2. ~/.config/glab-cli when that directory already holds a config.yml.
3. Otherwise $XDG_CONFIG_HOME/glab-cli — on macOS, ~/Library/Application Support/glab-cli.

There was no way to resolve that from the CLI, so users and tooling had to reimplement the lookup or guess. That matters when granting a sandbox write access to the file, when pointing an editor at it, and when reporting bugs.

glab config path prints the resolved file; --dir prints the containing directory. --dir exists because a write creates a temporary file in the directory and renames it over config.yml, so anything granting write access needs the directory, not just the file.

Output goes to stdout so it composes:

$EDITOR "$(glab config path)"

The path is reported whether or not the file exists yet, so it works before the first glab auth login.

Related Issues

Came out of investigating #8262 (closed) (sandboxed glab usage), but it stands on its own and does not resolve that issue.

How has this been tested?

Four tests via cmdtest.SetupCmdForTest, covering the default output, the --dir branch, a not-yet-created config file, and argument rejection (cobra.NoArgs).

Manually, against a real config:

$ glab config path
/Users/USERNAME/.config/glab-cli/config.yml

$ glab config path --dir
/Users/USERNAME/.config/glab-cli

$ GLAB_CONFIG_DIR=/tmp/whatever glab config path
/tmp/whatever/config.yml

make gen-docs run and the generated pages committed. Help output checked at a narrow terminal width to confirm the Long paragraphs reflow without orphan fragments.