feat(agent-manager): add keyboard shortcut tooltips and worktree hover cards (#516)

* feat(agent-manager): add keyboard shortcut tooltips and worktree hover cards

- Add TooltipKeybind to all agent manager buttons (tabs, new session,
  close tab, terminal, new worktree, delete worktree, promote to worktree)
- Add HoverCard popovers on worktree items showing branch name, base
  branch, session count, creation date, and navigation shortcut hint
- Show directional keybind hints only for directly adjacent items
  (tabs: ⌘←/⌘→, worktrees: ⌘↑/⌘↓)
- Resolve keybindings from package.json at runtime per platform
  (Mac symbols vs Windows/Linux Ctrl+key format)
- Send resolved keybindings from extension to webview via new
  agentManager.keybindings message instead of hardcoding shortcuts
- Prevent multiple HoverCards from appearing simultaneously via
  shared hover state lifted above the For loop
- Suppress HoverCard when hovering the delete button

* refactor: extract formatKeybinding and adjacentHint into testable modules

- Extract formatKeybinding into format-keybinding.ts (pure function,
  no vscode dependency, takes mac boolean parameter)
- Extract adjacentHint into navigate.ts as a reusable pure function
  for computing directional keybind hints
- Add tests for adjacentHint (12 cases) and formatKeybinding (13 cases)
- Simplify tab/worktree direction logic in AgentManagerApp to use
  adjacentHint instead of inline index arithmetic