Adopts main's round-19 features into the v2 stack: payload-budgeted
orchestration replay (#8992), sidebar row subscription leases (#9052),
tool group virtualization and scroll anchoring (#9106), repeated-command
and browser-group presentation, inline assistant citations (#9146),
per-cwd provider skills discovery (#8778), Claude composer skill
dispatch (#9128), grok health probe and model negotiation (#9154), and
the failed-tool thinking fallback (#9165).

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