chore: publish to stable channel (remove --unstable flag)

[PaulNewling]

What

Remove the --unstable flag from the block's prepublishOnly script in block/package.json.

Why

With --unstable, every CI release publishes the block without adding it to the stable channel, so the block only appears in the unstable channel. Removing the flag restores the default behavior: block-tools publish auto-adds the published version to the stable channel, so the next release reaches users on the stable channel.

The manual mark-stable script/workflow remains available as before.

Changes

• block/package.json: block-tools publish --unstable -r … → block-tools publish -r …
• Changeset: patch bump on the root block package so a release is cut.

[gemini-code-assist]

Code Review

This pull request removes the --unstable flag from the prepublishOnly script in block/package.json to publish to the stable channel by default, and adds a corresponding changeset file. There are no review comments, so I have no feedback to provide.

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