#9066 chore: delete four modules nothing imports

What & why

Nothing imports these four files. Each one is a delete-only change:

File	Lines	Evidence
apps/desktop/src/renderer/react-query/workspaces/useImportExternalWorktrees.ts	13	#4113 removed its last caller. The barrel exports only useImportAllWorktrees.
apps/desktop/src/shared/workspace-error-types.ts	30	No importer of the file, WorkspaceErrorCause or isWorkspaceErrorCause
apps/desktop/src/main/lib/build-channel.ts	12	No importer. auto-updater.ts has its own private copy of isPrereleaseBuild.
packages/host-service/src/trpc/router/workspace-creation/shared/start-point.ts	32	No importer of the file or buildStartPointFromHint

The importExternalWorktrees tRPC procedure stays. This PR removes only the unused client hook, so there's no API change.

How I tested it

• Grepped the symbol names and import paths across apps/ and packages/ (ts/tsx/json). The only hits are the files themselves.
• apps/desktop and packages/host-service typecheck pass

Opened by the daily cleanup automation.

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Summary by cubic

Deletes four modules that nothing imports, all delete-only with no behavior or API change. Each file's last caller was removed in earlier PRs, and the typecheck passes across apps/desktop and packages/host-service.

• Removes useImportExternalWorktrees, whose only caller was removed by #4113; the importExternalWorktrees tRPC procedure stays.
• Removes workspace-error-types.ts, build-channel.ts, and start-point.ts, whose types and functions have no importers.

Summary by CodeRabbit

• Chores
  • Removed several internal workspace-related interfaces and build-channel checks. No direct changes to the app’s user-facing behavior are specified.