Generate Cursor rules for Rill projects with template

[grahamplata]

Allows a user to generate Cursor rules for Rill projects with the CLI.

• Adding Cursor rules to an existing project
• Generating an empty project with Cursor rules

# Example adding cursor rules
rill init --template cursor
rill init <my existing project> --template cursor

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 - (Yes will need to be generated)
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[begelundmuller]

NOTE: Wiring up the actual Cursor instructions in a separate instructions API, using the same instructions we're using for our own agents and for Claude

[begelundmuller]

Integrated the actual Cursor rules and Claude Skills in this PR: #8615