Add interactive session switcher for navigating between subagent sessions

[rmk40]

Problem

When working with subagent sessions (e.g., tasks spawned by the task tool), navigation currently requires cycling through sessions one-by-one using <leader>right / <leader>left. This becomes inefficient when:

• Multiple subagents are spawned from a single parent session
• Subagents spawn their own subagents (nested hierarchy)
• You need to jump directly to a specific session (e.g., one awaiting permission input)

There's no way to see all related sessions at once or jump directly to a specific one.

Proposed Solution

Add an interactive session switcher dialog that:

1. Shows hierarchical view of parent and all descendant sessions (up to 3 levels deep)
2. Displays session status - busy spinner for active sessions, permission indicator for sessions awaiting user input
3. Allows direct navigation - select any session from the list instead of cycling through sequentially
4. Integrates with command palette - appears in suggested commands when subagents exist

Keybind

<leader>s (configurable via session_child_list)

UI Example

Switch Session
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Parent
  ● Main session title                              2m ago

Subagents
    explore task                                    1m ago
    code-review task                               45s ago

Subagents › explore
  ◉ nested-search task                             30s ago

• ● indicates current session
• ◉ (warning color) indicates session awaiting permission
• Spinner shown for busy/active sessions

Why This Is Useful

Direct selection is significantly more efficient than sequential cycling when you have 5+ subagent sessions and need to:

• Check on a specific task's progress
• Respond to a permission prompt in a nested subagent
• Return to the parent session after reviewing children

Implementation Approach

The implementation reuses existing patterns from the codebase:

• DialogSelect component for the selection UI (same as DialogSessionList, DialogTimeline)
• sync.data.permission[sessionID] for pending permission detection
• sync.data.session_status[sessionID] for busy state detection
• Category grouping for hierarchical display (same pattern as date grouping in DialogSessionList)

Style note

The implementation uses a let statement when walking up the session tree to find the root parent:

let current = currentSession()
while (current?.parentID) {
  const parent = sync.session.get(current.parentID)
  if (!parent) break
  current = parent
}

This is a legitimate use case where let is appropriate - the variable must be reassigned during iterative traversal. Per the style guide, let should be avoided where possible, but not prohibited when it's the natural solution.