chore: update cost table guidance

[ajkerrigan]

what

Updates sql_info reference documentation to point to our newer FOCUS-inspired tables, in an attempt to obviate CLAUDE.md / AGENTS.md instructions like this:

Cost Queries

Avoid these deprecated tables:

• account_cost
• resource_cost
• resource_cost_summaries

Instead, use the tables below that fit the use case.

Account Costs

• Daily granularity: account_focus_cost
• Monthly granularity: account_cost_monthly

Resource Costs

• Daily granularity: resource_focus_cost
• Monthly granularity: resource_cost_monthly

why

The FOCUS-styled tables are where we're moving in general. Queries against those tables are more performant, don't require dividing by 1000.0 to get "real" numbers, and leave some AWS bias behind. The additional joins make them more complex though, especially for new folks. Leaning on MCP should help with the transition.

testing

Tested these questions:

1. Please write a SQL query to smoke test Azure cost ingestion over the past 30 days
2. Show me the 10 most expensive resources this month

In 3 different scenarios:

1. Without this change, no explicit CLAUDE.md guidance
2. Without this change, CLAUDE.md guidance quoted above
3. With this change, no explicit CLAUDE.md guidance

I found that either the change or CLAUDE.md guidance causes the MCP server to produce "legitimate" cost queries. Without some sql_info.md additions for lookup tables like cost_source, account_id and resource_id, it is more likely to look for non-existent tables or infer too much (example: cost source 1 contains the word "azure" so I don't need to bother pulling account details).

docs

This is in a way a documentation change, but is also intended to align with updates to this Freshdesk page:

How do I interpret Stacklet's cost data?

[jtroup]

Thanks @ajkerrigan - I wonder if we want to mention something about cost settings/types too?