docs: use GLAB_DEBUG in the bug template and agent guide

Lee Seonghyeon · 이성현requested to merge
ihopenre-eng/cli:docs/glab-debug-env-var into main

What does this MR do and why?

IsEnvVarEnabled in internal/utils/utils.go prints a deprecation warning for any variable that does not carry the GLAB_ prefix. The bug report template asks reporters to run:

DEBUG=true glab mr list

so anyone following it is greeted by glab's own warning:

DEPRECATION WARNING: The environment variable DEBUG has been deprecated and
will be removed in future releases. Use GLAB_DEBUG instead.

glab --help and the published environment variable documentation already list only GLAB_DEBUG. This lines the two remaining references up with them: the bug report template, and the environment variable list in AGENTS.md.

internal/dbg keeps accepting both names, so nothing changes for anyone already using DEBUG.

Related issues

Closes #8401 (closed)

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