fix(enable): make import offer cancellable and fully best-effort

Address PR review feedback:

• Thread ctx into promptImportSelection and use form.RunWithContext so parent-command cancellation stops the prompt and is handled by handleFormCancellation.
• Never fail enable on a prompt/UI error: maybeOfferSessionImport is now void; prompt failures are logged, surfaced as a note, and skipped.
• Fix stale doc comment referring to the old claudeFixture name.