Add anti-guessing hint to execute_sql_query MCP tool description

[jtiisto]

What

Adds a three-line anti-guessing hint to the execute_sql_query tool description in the localdb MCP server, pointing clients to get_table_details before they write queries.

Why

The execute_sql_query docstring is surfaced verbatim as the MCP tool description that LLM clients read. Today it pitches the tool and lists a handful of example columns, but it never tells the model to confirm the schema first. The sibling tools already signpost the workflow forward — explore_database_structure and get_table_details both say "use me first" — yet the tool a model actually jumps to, execute_sql_query, has no backward pointer and no "don't guess" instruction.

With daily_health_metrics carrying 50+ columns (only a few of which appear in the examples), a memoryless client extrapolates the rest and fails:

• Wrong column name → hard error: Query execution failed: no such column…
• Wrong string value → silent 0 rows (no error), so the model wrongly concludes "no data."

The change

Insert immediately after the IMPORTANT: Only SELECT… line:

Column names are non-obvious (daily_health_metrics alone has 50+ columns) — call get_table_details(table_name) to confirm exact names before querying; do NOT guess.

This closes the explore_database_structure → get_table_details → query loop at the point of use — the one place the chain wasn't signposted.

Scope & safety

• Docstring-only — no logic or behavior change; the description re-exports on server reload.
• The column names referenced in the hint were verified against the current DailyHealthMetric model (57 mapped columns).
• No test asserts on this description; ruff format --check passes and the change introduces no new lint findings.