test: split monolithic script tests

[baskduf]

Summary

• Split the large tests/test_scripts.py suite into focused modules for manifests/docs, CI/release, coverage, findings, goals, and LiteLLM helpers
• Move shared test paths/loaders into tests/support.py
• Keep tests/test_scripts.py as a compatibility placeholder and update py_compile commands to cover tests/*.py
• Add import fallback so unittest discovery also works from outside the repository root

Test Plan

• python3 -m unittest discover -s tests -v
• cd $(mktemp -d) && python3 -m unittest discover -s /Users/wb/Desktop/FableCodex/tests -v
• python3 -m unittest tests.test_ci_release.CiReleaseTests.test_ci_workflow_validates_against_pinned_source -v
• python3 -m py_compile target scripts and tests/*.py; sh -n wrapper scripts
• python3 plugins/codex-fable5/skills/codex-fable5/scripts/fable_coverage.py
• git diff --check
• Compared old vs split test method names: old=32, current=32, missing=[]