[AThevon]

Release PR for 6.0.0. The features are already on main (#267, #270, #272, #274, #275); this carries the version and the README.

What's in it

• MARKETING_VERSION 5.13.0 to 6.0.0, and the matching Current version line in AGENTS.md, which check-docs.py verifies against project.yml.
• README corrections. The Codex text predates the work that made OpenAI a first-class provider, and three of its claims stopped being true: that menu bar segments and the popover remain Claude features, that Codex is enabled under Settings > General > Providers, and that the Claude onboarding requirements still apply. The menu bar and the popover cover both providers, the switch lives under Settings > Providers, and the wizard finishes with OpenAI alone. The feature list now presents the two providers as equals, the dashboard line counts four spaces (Studio had been missing since 5.9), the read-only rule is stated for both providers, and the troubleshooting table points at Reset connection.
• .gitignore covers Python bytecode from scripts/check-docs.py.

Verification

• The tree of this branch is identical to the release candidate that was tested: 913 tests, check-docs.py, Release build on Xcode 16.4.
• Upgrade path. A 6.0.0 build was launched on a real set of preferences stripped back to their 5.13.0 shape. Every 5.13.0 key came through untouched, popover and menu bar layouts included, byte for byte; nothing was lost; the only keys written were codexEnabled (auto-detected, false on a Claude-only machine) and popoverShowsProviderSwitch (its default). Onboarding stays completed, and the What's new screen is armed for the minor change.