feat(cli): export and import sessions (#11075)

* add session export slash command

* import session

* Fix slashCommands test: mock path.isAbsolute and history manager

The import of historyManager in slashCommands.ts triggers
core/util/paths.ts which calls path.isAbsolute — not provided by
the existing path mock. Add the missing path exports and mock
core/util/history.js to avoid filesystem operations in tests.

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Co-authored-by: Dallin Romney <dallinromney@gmail.com>