Add resume handoff guidance to session history export #43

Parent #4

Exported session markdown is portable context, but a reader opening it in a new folder has no in-file guidance: continuing in the original folder uses Pidex session history resume, while a new folder needs a fresh session that takes the export as context. Without that distinction, users can create an accidental new empty session or assume the markdown restores a native Pi archive.

Slice:

• Add a Continue elsewhere handoff section to sessionExportMarkdown output when the export has entries. It names the original working folder, distinguishes same-folder resume via Sessions history from new-folder continuation with the exported file as context, and states plainly the markdown is not a restorable Pi session archive.
• Keep empty exports free of the handoff section.
• Cover with unit tests in tests/unit/export-session.test.ts.

Acceptance: exported markdown for a non-empty session contains the handoff; empty exports do not; existing export content (title, context, tool input/output/status) is unchanged apart from the added section.