Add MCP tool for read-only database queries

[claytoncollie]

Problem

Inspecting WordPress data currently means either invoking wp db query through wp_cli (returns stdout, must be parsed) or opening Adminer in a browser. A typed read-only query tool gives agents a structured way to inspect data without leaving the agent loop.

Proposal

• query_database(siteId?, sql) → restricted to SELECT / SHOW / DESCRIBE / EXPLAIN; returns structured rows; rejects DML/DDL with a clear error

Write queries stay manual or behind explicit confirmation in a separate tool (out of scope for this issue).

Scope rationale

This overlaps with wp_cli (wp db query), but:

1. Server-side enforcement of read-only is safer than relying on the agent to remember
2. Structured row returns vs. stdout parsing is a real ergonomics win
3. Aligns with the safety direction already established in No server-side block on destructive WP-CLI commands #19 (block destructive WP-CLI server-side)

LocalApi additions

None strictly required — can run via the existing MySQL connection used by wp_cli, with a SQL parser to enforce read-only.

Acceptance criteria

• Rejects any statement that isn't SELECT/SHOW/DESCRIBE/EXPLAIN
• Returns rows as structured JSON, not stdout strings
• Handles multi-statement input safely (rejects, or runs only the first SELECT)
• Clear error when site is halted or DB unreachable

[iandunn]

This would be really helpful to cut down on nuisance approvals