fix(installer): honor XDG config home

[efegokdemir]

Summary

Honor XDG_CONFIG_HOME when the installer locates and updates the local gateway registration and mTLS client bundle.

Related Issue

Fixes #4042

Changes

• Resolve the target config directory from XDG_CONFIG_HOME, falling back to $HOME/.config when unset or empty.
• Pass the resolved config directory explicitly to target-user commands.
• Add installer coverage for unset, empty, set, and set-with-spaces values.

Testing

• bash tasks/scripts/test-install-sh.sh
• sh -n install.sh
• git diff --check

mise run pre-commit was not available because mise is not installed in the local environment.

Checklist

• Focused tests pass
• Shell syntax check passes
• Changes are limited to the installer and its focused tests
• Commit is signed off

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[efegokdemir]

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[efegokdemir]

recheck