Update rill init to be interactive

[TheRillJon]

Enable rill init to be an interactive tool for initializing a new Rill project, including:

• interactive menu
• install ai instructions
• copy demo project content (if using duckdb for olap)
• flags for all interactive options
• calls rill start but can be run in non-interactive mode to just create the files

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[begelundmuller]

@TheRillJon I pushed the following changes to this branch:

• Supports using --agent on an existing project (as long as no other flags are passed)
• Replaced "Start Rill?" prompt with printed instructions to run rill start
• Replaced --demo with --example (to mirror existing terminology elsewhere)
• Adds the available example project names in the command's long description
• Removed --non-interactive in favour of the existing global --interactive=false flag
• Removed the --force flag (it was ambiguous and didn't add much value; it will now always force agent instructions)
• Filters examples by the selected OLAP engine
• If an explicit project path is not given, it uses the current directory if it contains a Rill project (instead of prompting)
• Non-interactive mode without args errors instead of silently using "my-rill-project"
• Interactive prompting is all-or-nothing: if any flag is set, no prompts at all (assumes user made an active choice and wanted the defaults for the rest)

Let me know what you think!