[FEATURE]: improved visual position indicator for subagent sessions #9989

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

Problem
The header component facilitates navigation between subagent sessions but lacks visual context regarding the user's current position and the total number of sessions. Users cannot identify which specific subagent is active relative to others in the stack.

Proposed Solution
Enhance the header to include a position indicator and adjacent session metadata. This provides immediate orientation within the navigation cycle (Parent → Child 1 → Child 2 → Parent).

Technical Requirements
Modify the header component to derive the following state from existing session data:

• Current Index: children().findIndex(s => s.id === route.sessionID) + 1
• Total Count: children().length
• Adjacent Metadata: Retrieve names for children[index - 1] and children[index + 1] to provide context for navigation buttons.

UI Specifications

1. Indicator: Insert a "Current/Total" (e.g., "2 / 5") text element or a horizontal dot-based indicator.
2. Placement: Position the indicator between the "Subagent session" label and the navigation buttons.
3. Contextual Labels: Display the names of the previous and next agents near the navigation controls to clarify the destination of the "back" and "forward" actions.

Implementation Logic

const sessions = children();
const currentIndex = sessions.findIndex((s) => s.id === route.sessionID);
const total = sessions.length;

const prevAgent = currentIndex > 0 ? sessions[currentIndex - 1] : null;
const nextAgent = currentIndex < total - 1 ? sessions[currentIndex + 1] : null;