Parse Boolean action inputs as data

[Kewe63]

Summary

Parse Boolean action inputs as data and reject malformed values before any external command runs.

Changes

• Validate all Boolean-like inputs as exactly true or false.
• Treat an omitted maintenance_policy_terminate as false and make its default explicit.
• Replace command-position checks for actions_preinstalled and arm with quoted string comparisons.
• Enable maintenance termination only when its value is exactly true.
• Pass Boolean inputs through quoted environment variables instead of rendering GitHub expressions directly into shell source.
• Add regression coverage for valid values, empty/mixed/malformed values, flag behavior, and a benign marker executable that proves malformed input is never run.
• Run the regression test in a dedicated pull-request workflow.

Verification

• tests/boolean_input_validation_test.sh
• Sabotage check: the regression test fails against upstream/main, executes the benign marker there, and passes with this change without executing it.
• pre-commit run --all-files
• git diff --check

Compatibility and risk

Documented true and false values keep their intended behavior. Previously accepted malformed values now fail early; maintenance_policy_terminate: false no longer enables termination merely because the string is non-empty.

Fixes #74