feat: add Rivet release action

[bdbch]

No description provided.

[Copilot]

Pull request overview

This PR introduces a first-party Rivet GitHub Action to automate creating version PRs and publishing releases from GitHub Actions, and updates the CLI/docs/workflows to support that automated flow.

Changes:

• Added a Node-based GitHub Action (action.yml + action/index.js) that checks release state, opens/updates a version PR, and runs a publish command once ready.
• Extended rivet check with --json output for stable automation parsing.
• Updated release documentation and GitHub workflows to use the new action, plus added docs for action usage and action-release tagging.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
RELEASING.md	Updates the documented release flow to use an automated version PR + publish cycle on main.
packages/rivet/src/lib.rs	Wires CLI parsing for rivet check --json into the command dispatcher.
packages/rivet/src/commands/check.rs	Adds JSON output mode and refactors status computation/printing.
packages/rivet/src/cli.rs	Adds --json flag to the check subcommand.
apps/docs/content/docs/usage.mdx	Documents rivet check --json and the stable JSON status strings.
apps/docs/content/docs/meta.json	Adds the new “action” docs page to the docs nav.
apps/docs/content/docs/index.mdx	Links to the new GitHub Action guide.
apps/docs/content/docs/action.mdx	New guide describing setup/inputs/outputs for the Rivet GitHub Action.
action/index.js	Implements the action logic (check → bump PR → publish), outputs, and git/npm setup.
action.yml	Declares the action inputs/outputs and Node runtime.
ACTION.md	Repository-level documentation for consuming and releasing the action.
.github/workflows/release.yml	Switches release workflow to run on main and invoke the local action.
.github/workflows/action-release.yml	Adds a workflow to tag and publish versioned releases of the GitHub Action.

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