fix: resolve maintainers from repository root

[nightcityblade]

Summary

Resolve the MAINTAINERS file relative to the script's repository location.

Motivation

Fixes #595.

Target and compatibility impact

None; absolute and repository-root invocations now behave consistently.

Validation

• python3 -m unittest tools.test_get_maintainer (1 passed)
• python3 -m ruff check tools/get_maintainer.py tools/test_get_maintainer.py
• git diff --check

Checklist

• Commits include a DCO Signed-off-by line.
• Tests cover new behavior or the PR explains why no test is needed.
• User-facing changes include documentation or diagnostics updates.
• The change preserves the license boundary between the compiler and std/.

[LunaStev]

LGTM