feat(tui): add per-session tab indicator slot

Problem

V2 TUI plugins can contribute to shared surfaces such as the sidebar and prompt footer, but cannot attach compact metadata to individual session tabs. Integrations such as PR tracking need to show one status glyph for each tab's session without owning tab layout.

Proposed Direction

Publish a repeated typed slot such as session.tab.indicator with:

{
  sessionID: string
  orientation: "horizontal" | "vertical"
}

The host should mount it once per real session tab in both layouts and constrain it to one cell so plugin content cannot alter tab geometry, drag behavior, overflow, or scrolling.

Acceptance Criteria

• Available in vertical and horizontal V2 session tabs.
• Receives the associated session ID and orientation.
• Not mounted for the new-session affordance.
• Host-clipped to a stable one-cell footprint.
• Covered by slot typing and tab rendering tests.

Scope

This is a generic TUI plugin seam. It should not contain PR- or GitHub-specific behavior.