fix(tui): default tabs to global scope

[kitlangton]

What

Session tabs now use one global tab set by default. Users can still choose current directory in TUI settings when they want separate tab sets per working directory.

Before / After

Before: With tabs enabled and no explicit scope, each working directory had an independent tab set, and the settings dialog identified current directory as the default.

After: With no explicit scope, all working directories share the global tab set. An explicit tabs.scope: "cwd" continues to select per-directory storage, and the settings dialog identifies global as the default.

How

• Updates the runtime fallback in packages/tui/src/context/session-tabs.tsx to select global storage unless cwd is explicit.
• Updates the Tabs > Scope setting metadata in packages/tui/src/component/dialog-config.tsx.
• Adds a persistence regression test proving an unspecified scope writes the opened session into the global bucket.

Scope

This PR changes only the tab-strip scope default. The session picker default will follow in a separate PR.

Testing

• bun run test test/context/session-tabs.test.tsx
• bun typecheck from packages/tui
• Full workspace typecheck run by the pre-push hook
• bunx prettier --check src/context/session-tabs.tsx src/component/dialog-config.tsx test/context/session-tabs.test.tsx