Stabilize driver-agnostic core architecture

[piraz]

Summary

Stabilize sqlok around a driver-agnostic core based on database/sql.

Goals

• remove database driver dependencies from the core library
• remove driver-specific bootstrap code from the core codebase
• keep sqlok focused on query building, mapper behavior, session/unit-of-work behavior, and database/sql-based execution contracts
• define what remains in core versus what must stay outside the project

Scope

• refactor core code away from pgx-specific coupling
• neutralize CLI/example paths that currently open concrete drivers in core
• review schema loading and loader behavior for driver-agnostic boundaries
• clarify the future boundary for real database integration testing

Out of scope

• adding new vendor-specific adapters inside this project
• implementing external adapter projects in this ticket

Expected outcome

• sqlok core compiles and operates without bundling a concrete database driver
• architecture and code boundaries are explicit enough to continue the cleanup safely