Add database/sql resilience example.

[Mak-1911]

Fixes #60

Description

Adds a new example in examples/sql/ demonstrating how to use Resile with Go's standard database/sql package.

The example wraps db.ExecContext with resile.Do, retries, and a circuit breaker. It uses a small mock database/sql/driver that fails the first two calls and succeeds on the third call, so the example can run without SQLite, external services, or new dependencies.

Checklist

Please ensure your contribution meets the project standards:

• Code is formatted with go fmt.

• All tests pass (go test -v ./...).

• Architectural mandates (zero-dependency, type safety) are respected.

• Tested with:

  go run ./examples/sql
  go test -v ./...

Additional Context

This example uses a mock SQL driver instead of SQLite to keep the example dependency-free while still exercising the standard database/sql API. Although I can implement the SQLite if the project demands.