docs: document MCP status tool

[LeslieWylie]

Summary

Document the MCP get_status tool as the first connection check and make catalog-size guidance reflect the live server state.

Changes

• replace the hard-coded "1000+ sources" claim with wording tied to the live FirstData catalog
• document get_status, including its version, tool-list, catalog snapshot, and runtime-data signals
• update the MCP tool count from 5 to 6
• tell agents to call get_status before choosing a search tool or diagnosing a connection

Why

Catalog contents change over time. Agent instructions should read the current snapshot metadata instead of repeating a stale source count.

Validation

• documentation-only change
• repository check-secrecy workflow passes