fix(tui): hide single session tab

[kitlangton]

What

Hide the opt-in session tab strip when only one real tab is open. The strip appears once a second tab is added, where tab navigation becomes useful.

Before / After

Before: Enabling session tabs showed a full tab row even when the working set contained only the current session.

After: Zero or one tab uses the full terminal height. Opening a second tab reveals the strip; closing back to one hides it again.

How

• Update the SessionTabs render guard in packages/tui/src/app.tsx to require more than one real tab.
• Keep tab persistence, unread state, commands, and keyboard navigation unchanged.

Scope

This only changes strip visibility. It does not alter which sessions belong to the tab working set or when tabs are created and closed.

Testing

• bun typecheck in packages/tui
• bun run test in packages/tui (562 pass, 5 skip, 0 fail)
• Push-hook workspace typecheck (33 tasks passed)
• bunx prettier --check packages/tui/src/app.tsx
• git diff --check