[Feature]: Agent Session Import with Compatibility Preview

## Background

Agent sessions can be exported, but users cannot import an external or previously exported session as a verifiable Agent workspace. This blocks migration between devices and tools and makes file references or incomplete task state difficult to assess safely.

## Goal

Users can import a supported Agent-session format after previewing its task, messages, referenced files, and compatibility findings.

## Spec

- Define supported, versioned Agent-session import formats and validate a selected file before changing local data.
- Preview the task objective, message count, model and runtime metadata, referenced files, and unresolved actions.
- Identify missing, inaccessible, or conflicting file references and let users map, exclude, or keep them unresolved.
- Keep imported sessions separate from existing sessions unless the user explicitly chooses a supported merge behavior.
- Apply no changes until final confirmation, then provide an import summary with skipped and failed items.

## Verification

- Select a valid exported Agent session → the preview shows its task, messages, and file references before import.
- Import on a device where one referenced workspace is missing → the user can remap or exclude that reference without losing the remaining session.
- Select an unsupported or corrupted file → validation stops before any session is created and explains the incompatible part.
- Cancel from preview → no Agent session, message, workspace, or file reference is written.

## Related

#17904

Review related issue #17904 and the existing Agent-session export flow first; the issue does not identify implementation files or tests. Define the supported versioned format, preview and validation flow, cancellation guarantees, reference handling, and import summary, then verify valid, missing-reference, corrupted, and cancelled imports.

Written by the indexing model from the issue text.