This PR adds a project-level chatbot behind the chat feature flag. The dashboard-level chatbot has been retained and moved behind the dashboardChat feature flag.

Closes APP-227

Note: The system prompt needs work, but I'll take prompt improvements in a follow-on PR after we have our eval system in place.

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!