[codex] Drive changelog releases from towncrier

[adamtheturtle]

Summary

This switches release changelog handling to the same towncrier-based flow used by literalizer:

• replace the manual Next changelog section update with towncrier build
• generate GitHub release notes from the same news fragments
• add the towncrier config, template, and newsfragments/ directory
• render unreleased fragments in docs where the project has Sphinx docs
• migrate any existing Next entries into initial news fragments

Validation

• uv run --extra=release towncrier build --draft --version 2099.01.01
• uvx --from actionlint-py actionlint .github/workflows/release.yml
• uv run --extra=dev pyproject-fmt --check --no-print-diff pyproject.toml where the repo has a dev extra
• repo pre-commit / pre-push hooks run by git commit and git push