feat(desktop): import external sessions

[likun666661]

Summary

• add bounded Runtime Host operations for external Agent discovery, Session listing, and import
• keep source-specific transcript reads and conversion inside Runtime Host so raw histories never cross Desktop IPC
• expose a typed Electron main/preload bridge and a localized Desktop import picker
• create each import atomically through the existing ExternalSessionImporter; importing the same source again creates an independent copy

Part of #2499 (Desktop/IPC phase).

Architecture

Desktop renderer
  -> typed Electron IPC (adapterId + sourceSessionId only)
  -> Desktop Runtime Host client
  -> bounded external-session.* operations
  -> ExternalSessionAdapterRegistry / source adapter
  -> ExternalSessionImporter
  -> atomic createImportedSession

The external source remains read-only. Catalog pages are capped by item count and encoded byte size, unsupported/internal Agent threads remain the adapter responsibility, and commit uncertainty drains the Host instead of silently retrying. The closed Runtime Host operation schema advances compatibility epoch 5 -> 6.

Desktop behavior

• add an Import conversation action to the Session sidebar
• detect supported local Agents and page their root Sessions
• optionally include archived Sessions
• show localized loading, empty, unavailable, retry, and import-failure states
• upsert and navigate to the imported Maka Session on success

Verification

• Storage, Runtime Host, UI, and Desktop typechecks
• Runtime Host protocol, coordinator, composition, and catalog regression tests
• Desktop IPC and candidate lifecycle tests
• UI import-action regression test
• Desktop preload and renderer production builds
• Biome, copy, accessibility, console, and git diff --check checks