`openspec init` cannot install agent skills in repositories using external stores

Problem

When a code repository points to an external OpenSpec store, openspec init refuses to run before generating agent integrations.

This leaves an onboarding gap: the store can be correctly configured, but a developer cannot use init to install the OpenSpec workflow skills needed by their coding agent without first removing the store declaration.

Our use case is to keep specs and changes in a shared planning repository while using OpenSpec workflows from the code repository.

Steps to reproduce

1. Register a valid external store named shared-planning.
2. In a separate repository with no agent integrations or local planning directories, create openspec\config.yaml containing:

   store: shared-planning
   

3. Run openspec init.

Actual behavior

Initialization stops before installing skills:

Remove the store: line first to convert this repo to a local OpenSpec root.

Expected behavior

Install agent skills while preserving store: and keeping specs and changes in the external repository, without creating a local planning root.

An integrations-only mode could address this. Global installation, proposed in #752 is another potential option.

Versions

Reproduced on OpenSpec 1.12.0, Windows, GitHub Copilot CLI. The same guard exists in the [1.13.0 source](https://github.com/Fission-AI/OpenSpec/blob/v1.13.0/src/core/init.ts), confirmed by source inspection—not a runtime reproduction on that version.