[FEATURE]: Export Options: configurable defaults or remember last-used values

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The TUI Export Options dialog (from /export) always opens with hardcoded defaults: Include thinking, Include tool details, and Include assistant metadata checked, Open without saving unchecked. The values are passed as literal arguments to DialogExportOptions.show(...) from the session export handler in packages/tui/src/routes/session/index.tsx — they are neither configurable nor remembered from the previous export.

This is a follow-up to #33263, which was auto-closed by the stale bot after 60 days but is still relevant.

Proposals (either would solve it):

1. Remember last-used values for the current session (or persist them), like most apps' export dialogs.
2. Config keys, e.g.:

{
  "export": {
    "thinking": true,
    "toolDetails": true,
    "assistantMetadata": true,
    "openWithoutSaving": true
  }
}

Use case: I almost always want Open without saving — pipe the transcript straight into $EDITOR without dropping a file into the project cwd — so every export currently costs an extra tab → space toggle before confirming.