mecatui: hide session ID by default and expose it through /session during runs

Problem

mecatui currently exposes the session ID/short handle by default in both its in-app header and terminal-title composition. This consumes scarce title/header space and makes the identifier visually prominent even when the user does not need it.

When the agent is busy, however, the user needs a reliable way to retrieve the full session ID for support, reconnect/recovery, or cross-client operations. The /session command should remain usable while a run is active so the ID can be captured without waiting for the agent to finish.

Requested behavior

1. Remove the session ID/short handle from the default in-app header.
2. Remove the session ID/short handle from the default terminal-title composition.
3. Ensure /session can run while the agent is busy and renders the current session’s ID.

Acceptance criteria

• During an active agent run, entering /session returns/displays the current session ID without cancelling, queueing behind, or otherwise disrupting the active run.
• The default in-app header does not display the session ID or short handle.
• The default terminal title does not include the session ID or short handle.
• Session identity remains available through /session for users who need it.
• Add regression coverage for the busy-state /session behavior and the default header/title presentation.

Scope

This is intentionally about default presentation and session-ID discoverability. It does not change session identity, recovery semantics, or the terminal-title OSC behavior.