[FEATURE]: Show parent and child current context usage in the TUI sidebar #20644

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I want the TUI sidebar Context panel to show current context usage for a parent session and its direct child sessions.
In version 1.3.13, the sidebar only shows context usage for the currently selected session. When working in a parent session that spawns subagents, it is hard to understand the full context footprint of the overall task from the parent view.
Proposed behavior:

• In a parent session, show the combined current context usage for the parent and its direct child sessions in the sidebar.
• Also show a per-session breakdown so the user can see which child session is using how much context.
• Keep the existing single-session behavior unchanged when there are no child sessions.
• This should be a current context view, not cumulative lifetime token usage.
  Why this would help:
• It makes subagent-heavy workflows easier to monitor from the parent session.
• It gives better visibility into family-level context pressure without forcing the user to switch into each child session.
• It keeps the information in the existing sidebar context area, so the feature is discoverable and low-friction.
  Possible implementation direction:
• Extend the TUI plugin state API so sidebar plugins can inspect loaded session relationships.
• Update the existing sidebar context plugin to aggregate the parent session and its direct children.