Distribute pull-up as a standalone executable #32

Context

#26 introduces YAML configuration for built-in jobs, and #27 allows custom jobs to run as external commands. These are preparatory steps toward distributing pull-up as a single executable without requiring a separate Node.js installation.

Once these alternatives are available, discontinue support for pullup.config.ts. Built-in job configurations should move to YAML, and custom JavaScript/TypeScript callbacks should move to external commands.

Proposed work

Depends on #26 and #27.

• Publish a migration guide from pullup.config.ts to YAML and external commands, including before-and-after examples and runtime requirements for custom commands.
• Discontinue support for pullup.config.ts and document the breaking change.
• Package the CLI, its dependencies, and its runtime into a single executable for each supported platform.
• Publish platform-specific binaries through GitHub Releases.
• Document supported platforms and how to download and run the executable.

Acceptance criteria

• Built-in jobs configured through YAML work without installing Node.js or a JavaScript package manager.
• Custom jobs work through external commands. Users provide any runtimes required by those commands.
• Existing pullup.config.ts files produce a clear migration error instead of being silently ignored.
• sync, check, and sync --dry-run behave consistently across the standalone and npm distributions.
• Verify the binaries in environments without Node.js installed.