Round-16 rebase onto main (5 commits, headlined by #8236 file
attachments in the client). Main's upload-aware send path — capability
probes, upload await/retry gating, uploaded-ref turn attachments with
dataUrl fallback, draft release on success — is woven into the v2
dispatch flow, timeline user rows render file attachments as download
links with the ChatView download handler, and the provider settings
editor cleanup keeps the branch's environment-field rows. The codex
feedback client flow stays unported, replay testkit configs gained
main's environmentThemesDir, and the rpc/settings/docs unions carry
both sides.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>