refactor: share FableCodex state helpers

[baskduf]

Summary

• Extract shared Codex Fable5 state, locking, JSON, and event helpers into codex_fable_state.py
• Update goals/findings ledgers to use the shared helper module
• Strengthen CI workflow validation so the pinned FABLE-5 fetch output and coverage --source input remain semantically linked
• Add the new shared helper to CI, contributing, and release py_compile commands

Test Plan

• python3 -m unittest discover -s tests -v — Ran 34 tests, OK
• External cwd discovery: cd $(mktemp -d) && python3 -m unittest discover -s /Users/wb/Desktop/FableCodex/tests -v — Ran 34 tests, OK
• python3 -m py_compile for target scripts and tests/*.py
• sh -n wrapper scripts
• python3 plugins/codex-fable5/skills/codex-fable5/scripts/fable_coverage.py — matrix rows 71, valid
• git diff --check
• Wrapper smoke in temp dir for codex-fable5 goals create/next/checkpoint/status
• Parallel independent review agents: no blocking findings after including the new shared module in git

Review Notes

• The prior blocking finding was that codex_fable_state.py was untracked. This PR includes it in the commit.
• Non-blocking review note: the shared-helper identity test intentionally verifies this refactor's goal that goals/findings use the same helper objects.