Re-applies the deltas that mid-stack blob reverts discarded, and merges
main's work into the v2-owned surfaces:

- keybindings: main's STATIC_KEYBINDING_COMMANDS rename plus both new
  commands (rightPanel.toggleMaximized alongside threadPanel.toggle)
- OpenInPicker: main's remote-open/SSH routing and favorite-editor
  shortcut layered onto the branch's panel/toolbar variants; the
  extracted shouldShowOpenInPicker now takes remoteOpenMode
- ChatMarkdown: main's bare-filename resolver (#6297) ported into the
  branch's module-level component factory, plus #4133 title-attribute
  stripping on links and images
- ComposerPrimaryActions: main's #4781 model (stop stays reachable, send
  joins it when Enter-to-send is unavailable) carrying the branch's
  steering send button
- ComposerPendingUserInputPanel: main's collapsible redesign with the v2
  RuntimeRequestId and responseCapability gate
- ChatComposer: main's oversized-prompt submission guard wrapping the
  branch's dispatch-mode send
- preview shell: main's container-aware width clamp ported into the
  branch's usePreviewPanelInlineSize hook
- MessagesTimeline/Sidebar: main's day-aware timestamps, code-font tool
  bodies and provider accent badges on the v2 runtime shell
- index.css: main's @variant dark migration (#6381) replaces the branch's
  standalone .dark block
- contracts: main's send-turn image mime allowlist re-homed to
  chatAttachment.ts, where v2 keeps the other send-turn limits

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