pre-commit hook should run make lint when available

[akuzminsky]

Problem

The managed pre-commit hook in modules/plain-repo/files/pre-commit only runs:

1. terraform fmt -check -recursive
2. terraform-docs .
3. Trailing newline check

It does not check Python formatting (black --check) or any other linting. Repos that have a make lint target (which includes black --check, pylint, etc.) can pass the pre-commit hook locally but fail the CI lint step.

How we discovered it

In infrahouse/terraform-aws-actions-runner#87, a Python file that needed black reformatting passed the pre-commit hook and was pushed. CI's make lint caught it.

Proposed fix

Add a step to the pre-commit hook that runs make lint if a Makefile with a lint target exists:

# 4. Run project linter if available
if [ -f Makefile ] && grep -q '^lint:' Makefile; then
    echo "🔍 Running project linter..."
    if ! make lint; then
        echo "❌ Linting failed"
        echo "   Run: make lint"
        exit 1
    fi
    echo "✅ Linting passed"
fi

This would cover black, pylint, terraform fmt, and anything else a repo puts in its lint target — and is a no-op for repos without one.

Note: if make lint already includes terraform fmt -check, the existing step 1 becomes redundant but harmless. Could be deduplicated later.