Centered session action dialogs

[gadflysu]

Goal

Add a centered session action dialog in interactive picker mode before launching a selected agent session.

The first use case is a resumable-session confirmation dialog that shows the session metadata aps already knows, including agent, title, session ID, directory, time, turns/messages, and data/source path where available.

Plan

Detailed plan: docs/agent/plan-issue-48-centered-session-dialogs.md

Requirements

• Use the existing Bubble Tea + Lipgloss rendering model.
• Do not introduce huh, ultraviolet, or other new UI dependencies for this task.
• Keep launcher responsible only for final process replacement; do not launch agents from picker UI code.
• Keep the bottom status bar reserved as the final terminal row and do not cover it with dialog content.
• Route keys to the active dialog first so enter/esc do not also affect the underlying picker.
• Make the dialog structure reusable for future non-resumable history/cleaned sessions from Display cleaned Claude history sessions as non-resumable rows #45.

Scope

• Implement a small picker-owned dialog state and renderer.
• Show a centered confirmation dialog before returning the chosen session from picker mode.
• Add tests for key routing, confirm/cancel behavior, width-safe rendering, and status-bar last-row invariants.

Non-goals

• Do not implement Display cleaned Claude history sessions as non-resumable rows #45 history/cleaned session discovery in this issue.
• Do not migrate the UI to Bubble Tea v2, Lipgloss v2, or Ultraviolet.
• Do not replace the existing picker/list/preview architecture.
• Do not change list-mode output.