Model pickers: show where the list came from; refresh the Codex fallback (#1616)

* feat(ai): show where model lists come from; refresh the Codex fallback

- CODEX_FALLBACK_MODELS now matches codex 0.155.1 model/list (GPT-6-Sol default).
- Discovery captures the CLI version once (claude --version; codex initialize
  userAgent) and /api/ai/capabilities exposes it as optional toolVersion.
- Claude/Codex model pickers show a muted source hint when toolVersion is sent.

* fix(ai): publish the Codex version with the model list; tighten cliVersionFrom

- Codex toolVersion is set in the same finally step as the discovered models,
  so a capabilities answer mid-discovery never pairs a version with the
  still-fallback list (which made the picker claim the built-in list).
- cliVersionFrom reads the line naming the tool (claude: /claude code/i), else
  the first non-empty line, and refuses an over-long prerelease suffix.

* feat(guide): prefer gpt-6-luna as the Guided Review Codex default when offered

For a user with no saved guide Codex pick, resolve to gpt-6-luna when the
Codex catalog offers it, else Codex's own marked default. Resolution-only
(PREFERRED_GUIDE_CODEX_MODEL is never written to the cookie), so saved picks
win; review and Code Tour defaults are unchanged.