Add hard row limits for AI metrics and SQL queries

[begelundmuller]

This PR reduces the default and max row limits per query_sql and query_metrics_view tool call. It also updates the tool descriptions and agent prompts to discourage loading large datasets.

The row limits are configurable with the following config environment variables:

• rill.ai.default_query_limit (default: 25)
• rill.ai.max_query_limit (default: 250)

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!