Dispatch internal version sync after releases

[chailandau]

Why

Keep the Ghost dependency pins in squareup/agents and squareup/ghost-internal synchronized with each published Ghost release.

What

• Mint a repository-scoped sq-ghost-bot installation token after a successful publish
• Send a bump_ghost repository dispatch to squareup/agents with the published version
• Skip the integration cleanly on forks and when credentials are not configured
• Use repository_dispatch so the app does not need Actions permission

Setup

This expects the following Actions configuration on block/ghost:

• Variable: GHOST_SYNC_VERSION_APP_ID
• Secret: GHOST_SYNC_VERSION_PRIVATE_KEY

The receiving workflow in squareup/agents should land before this PR is made ready for review.

Testing

• git diff --check passed
• Local repo checks could not run because dependencies are not installed; install was blocked by the corporate dependency-confusion filter

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