[FEATURE]: env flag to disable global config loading (`OPENCODE_DISABLE_GLOBAL_CONFIG`)

[kynnyhsap]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

This recent PR #8093 added env flag to disable loading project config, but it would also be very useful to do the opposite - disable GLOBAL config loading.

Use Case

Creating project agent configurations - global config messes with local configuration.

Usage

OPENCODE_DISABLE_GLOBAL_CONFIG=true opencode - will load project config but ignore global.

PR with implementation: #10013

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Config Flag to mark part of the global config as non-overridable per project - Project level permissions should not be able to set bash permissions #7136: Config Flag to mark part of the global config as non-overridable per project

Both issues address global vs project config precedence, though from different angles. #7136 proposes marking global config as non-overridable, while this issue proposes disabling global config entirely.