feat(mecatui): show the active session title in stock status lines

Goal

The session title now exists as authoritative session metadata and drives the window title, but the stock mecatui status-line templates should also expose the active title without requiring custom StatusML.

Acceptance criteria

• Identify every built-in/header/footer stock status-line template and show the active session title where it is useful and width-safe.
• Preserve existing status-line layout, truncation, accessibility, and no-title fallback behavior.
• Use the authoritative active-session title/provenance projection; do not duplicate title derivation in the status-line renderer.
• Update deterministic UI/status-line tests and goldens.
• Document the stock-template behavior and customization interaction in the appropriate user-facing status-line documentation.