feat(sql): extract dialect interface, then add guarded MySQL engine

[jamiesun]

Background

internal/sqlsafe currently hardcodes PostgreSQL + SQLite paths (classify.go / classify_sqlite.go / postgres.go). MySQL is the highest-frequency next engine, but adding it naively risks combinatorial explosion across classify → gate → explain → backup → execute.

Plan

1. Refactor first: extract a Dialect interface (classification quirks, identifier quoting, read-only session mechanism, EXPLAIN/row-estimate strategy, backup strategy, client invocation) with Postgres/SQLite as the two implementations. Zero behavior change, existing tests must stay green.
2. Then MySQL: implement the dialect via remote mysql client — fail-closed classification (block multi-statement, INTO OUTFILE, LOAD DATA, stored-procedure/DELIMITER forms, writable SET GLOBAL…), read-only via SET TRANSACTION READ ONLY, DML backup via CREATE TABLE … SELECT or CSV snapshot under explicit locking, credential discovery parity with the Docker/env-file flow, run-mode command-position block for mysql/mariadb clients redirecting to sshx sql.

Acceptance

• Dialect interface refactor lands separately with no behavior change
• MySQL engine passes the same acceptance matrix rows as Postgres/SQLite (happy path, blocked classes, backup-before-mutate, restore drill)
• Run-mode guard extended to mysql client binaries