feat: branch switcher with switch and delete in diff pane - #2

- 4-page onboarding (Welcome, BYOK, Agents, Configure) gating the app;
  completion persisted to runtime-state.json
- TempestLogo wordmark component; landing screenshots bundled via Vite
- Global agent availability store (agentAvailability.ts); NewSessionMenu
  switches to useSyncExternalStore instead of per-open probe loop
- Sub-session persistence: ::sub:: key namespace, subSessionKey helpers,
  parentPath on WorktreeSession; restoreAll Phases 1/3/5 restore
  sub-sessions across restart
- Ghost-parent restore in launchTerminalWorktree so overview tab nesting
  is correct on first click
- Custom 'Use existing' branch dropdown in naming modal with worktree,
  remote, and open badges
- Sub-session naming ({AgentName} #2/#3) and sidebar branch badge pill
- git_list_branches switched to git branch -a; BranchInfo gains
  is_remote and is_worktree fields; remote branches shown in diff pane
  branch switcher (muted color, no delete button)

Co-authored-by: Tempest <tempestai.dev@gmail.com>

The #N suffix counted every live session at the destination path, so a
first Claude was born "Claude #2" whenever a terminal, diff or preview tab
happened to be open there. It also ignored closed sessions, so re-opening
a ghost and spawning another could mint a second "Claude #2".

Both spawn paths now share uniqueSessionName(), which collects the names
actually in use at that path -- live rows plus the persisted ghosts -- and
takes the first free "Claude", "Claude #2", "Claude #3". The suffix is
still only a default; renaming a session is unaffected.

Co-authored-by: Tempest <tempestai.dev@gmail.com>