[codex] Prepare 0.3.1 release

[baskduf]

Summary

• Bump codex-fable5 plugin version to 0.3.1.
• Move current unreleased changes into the 0.3.1 - 2026-06-15 changelog section.
• Add a regression check that the plugin manifest version is documented in the changelog.

Release Notes

This is a patch release for the goal ledger recovery fix, python3 documentation/CI updates, and marketplace path validation test hardening.

Verification

• python3 -m unittest discover -s tests -v
• python3 -m py_compile plugins/codex-fable5/skills/codex-fable5/scripts/codex_goals.py plugins/codex-fable5/skills/codex-fable5/scripts/fable_coverage.py plugins/codex-fable5/skills/codex-fable5/scripts/make_litellm_config.py tests/test_scripts.py
• python3 plugins/codex-fable5/skills/codex-fable5/scripts/fable_coverage.py
• git diff --cached --check

Not Included

• No release tag has been created yet.
• No GitHub Release has been published yet.